How daylight affects your energy

You may have slept for roughly the same amount of time as usual and still feel slow, sleepy, and mentally foggy throughout the day. Sometimes the difference is not only how long you slept, but how much light your body has been exposed to and at what time of day that light reached your eyes.

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Daylight is one of the body’s strongest time signals. It helps the brain distinguish morning from evening, influences alertness in the moment, and helps set the circadian rhythm so that sleep, hormones, body temperature, and energy levels follow a more predictable pattern. The effect depends mainly on light reaching the retina, not sunlight touching the skin.

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The eyes register more than what you consciously see

The retina contains the cells needed for vision, but it also contains light-sensitive nerve cells involved in regulating the circadian rhythm. These cells respond particularly strongly to light in the blue-green part of the spectrum and send information to the brain’s main biological clock.

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This master clock is located in the suprachiasmatic nucleus of the hypothalamus. It helps coordinate several rhythmic processes in the body, including:

• sleepiness and alertness

• melatonin secretion

• body temperature

• cortisol rhythm

• appetite and digestion

• mental performance

• timing of physical recovery

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The internal clock runs on an approximately 24-hour cycle, but not always with perfect precision. It therefore needs regular adjustment from environmental signals. Light is the strongest external signal for this process.

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Morning light tells the body that the day has begun

When the eyes are exposed to bright light in the morning, the brain receives a signal that the biological day should begin. Depending on the timing, morning light can help shift the circadian rhythm earlier, making it easier to feel alert earlier in the day and sleepy at a more suitable time in the evening.

Morning light may be particularly useful for people who:

• take a long time to feel awake

• go to bed and wake up late

• have an irregular sleep schedule

• spend most of the day indoors

• work in dimly lit rooms

• struggle to fall asleep at the desired time

• feel most alert late in the evening

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The effect is not so simple that a few minutes outdoors immediately removes all tiredness. Morning light has both an acute and a longer-term effect. It may increase alertness that morning, while repeated exposure at roughly the same time can also help stabilise the circadian rhythm.

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Research on light and sleep suggests that earlier light exposure, especially in the morning, is associated with an earlier circadian phase and may support better sleep-related outcomes. The exact effect depends on light intensity, duration, timing, and individual sensitivity.

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Outdoor light is usually far stronger than indoor light

A room can feel well lit enough for reading and working while still providing relatively little light compared with the outdoors.

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Even on an overcast day, outdoor light can be much brighter than the lighting in a typical office or living room. Under clear skies, the difference may be enormous.

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This is one reason sitting next to a window does not always produce the same effect as going outside.

How much light actually reaches the eyes depends on:

• cloud cover and season

• time of day

• how long you stay outside

• whether you wear sunglasses

• the direction of your gaze

• nearby buildings, roofs, and vegetation

• your distance from the window

• the strength of indoor lighting

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There is no need to look directly at the sun. Doing so can damage the eyes and is not necessary for a strong circadian signal. Ordinary outdoor exposure with the eyes open is sufficient.

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Light can increase alertness directly

Daylight does not only affect when you sleep. Bright light can also have an immediate activating effect while you are exposed to it.

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Many people feel more awake, less sleepy, and better able to concentrate after going outside.

Systematic reviews suggest that daytime light exposure can improve subjective alertness and, in some studies, attention and cognitive performance. The effect is not identical in every study, but higher light intensity and light containing more short wavelengths often appear to produce a stronger alerting response.

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The effect may be particularly noticeable when:

• you have just woken up

• you have spent a long time in a dark room

• your work is repetitive

• you experience an afternoon energy dip

• you have had very little outdoor exposure

• daylight suddenly becomes stronger after grey weather

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Light does not replace sleep. Someone with substantial sleep deprivation may temporarily feel more alert in bright light without reaction time, attention, and judgement being fully restored.

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Melatonin should be high at night, not all day

Melatonin is often described as a sleep hormone, but it is more accurately understood as a signal of biological darkness.

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Its production usually increases in the evening as light levels fall, remains high during the night, and decreases towards morning.

Bright light reaching the eyes can suppress melatonin. In the morning, this is usually useful because it helps end the biological night. In the evening, the same mechanism can shift the signal in an unwanted direction.

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This means that the timing of light matters just as much as the amount.

Time of day	Typical effect of bright light
Morning	Can increase alertness and shift the rhythm earlier
Middle of the day	Supports alertness and a clear daytime signal
Late afternoon	Can support activity, but the effect varies
Late evening	Can delay melatonin and sleep onset
Night	Can strongly disturb sleep and circadian timing

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Light from lamps, screens, and other sources is usually weaker than daylight, but evening exposure occurs at a time when the body is more sensitive to signals that extend the day.

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Strong or prolonged light exposure late in the evening can therefore delay sleepiness and make it harder to fall asleep at the desired time.

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Good daytime light can make sleep easier at night

Daytime energy is closely connected to sleep the night before, but also to how the body is prepared for the following night.

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A clear contrast between a bright day and a darker evening helps the circadian system organise sleep and wakefulness.

People who receive little light during the day and a lot of light in the evening may give the body unclear timing information. The result may be sleepiness during the working day and greater alertness when they actually want to go to bed.

A clearer daily light pattern looks more like this:

• plenty of light early and during the middle of the day

• regular outdoor activity

• gradually dimmer light towards evening

• limited bright light shortly before sleep

• a dark bedroom at night

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Daylight does not guarantee good sleep. Pain, stress, caffeine, illness, sleep apnoea, working hours, and many other factors can interfere with sleep.

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Light is still an important and often overlooked factor that can be adjusted in daily life.

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Body temperature also follows a daily rhythm

Core body temperature changes naturally across the day. It is usually lowest towards the end of the night and rises as the body enters the biological day. It then begins to fall again towards evening.

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This rhythm is closely linked to alertness. As body temperature rises during the morning, many people feel more awake. As it begins to fall in the evening, sleepiness increases.

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Light influences this pattern through the circadian system. Morning light can help provide a clearer starting signal, while bright light late in the evening can delay the temperature drop and the onset of sleepiness.

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This is also one reason light and physical activity often work well together. A morning walk provides:

• light exposure to the eyes

• increased muscle activity

• greater heat production

• increased circulation

• a clear shift from rest to activity

• a mental change of environment

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It is therefore difficult to determine how much of the improved energy after a walk comes from light alone. Movement, fresh air, environmental change, and expectations also contribute.

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The combined effect can still be practically useful.

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Winter darkness provides weaker time signals

In northern countries, daylight varies greatly across the year. During winter, many people wake before sunrise, work indoors during the brightest hours, and return home after dark.

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The total amount of light exposure can therefore be much lower than in spring and summer.

This may contribute to:

• more difficult waking

• greater daytime sleepiness

• a later circadian rhythm

• reduced energy

• increased sleep need in some people

• lower activity levels

• changes in mood

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For most people, the seasonal change is moderate. For some, it becomes more pronounced and may be part of seasonal depressive symptoms.

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Persistent low mood, loss of interest, marked functional decline, or other depressive symptoms should not be explained by lack of daylight alone.

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It is also important to distinguish the circadian effects of daylight from vitamin D.

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Vitamin D is produced in the skin in response to ultraviolet B radiation, while circadian timing is influenced mainly by visible light reaching the retina.

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A person can therefore receive a strong circadian light signal through the eyes without producing much vitamin D in the skin.

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Sitting by a window helps, but may not be enough

A window allows daylight into the room and can make the workspace brighter. People sitting near windows usually receive more light than those working deep inside a building.

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However, light intensity falls quickly with distance from the window.

Glass and building design also affect how much light enters. The view may appear bright even when the light level reaching the eyes is much lower than it would be outdoors.

A practical combination may include:

• placing the workspace near a window

• taking short outdoor breaks

• using adequate indoor lighting

• keeping the environment bright in the morning

• reducing light levels in the evening

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A short outdoor break can therefore provide a clearer light signal than a much longer period beside a window, especially on a bright day.

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Screens are not the same as daylight

Phones, tablets, and computer screens send light directly towards the eyes, but their intensity is usually far lower than outdoor daylight.

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Screen light during the day cannot therefore simply replace time outdoors.

In the evening, the situation is different. The surroundings are darker, the pupils may be larger, and the circadian system is biologically prepared for reduced light.

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Screen use may also keep the brain active through work, notifications, social interaction, and stimulating content.

The evening effect depends on:

• light intensity

• distance from the screen

• duration

• timing

• the screen’s light spectrum

• the surrounding room light

• how stimulating the content is

• individual circadian sensitivity

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It is therefore misleading to reduce the issue to blue light alone.

Night mode and warmer screen colours can reduce short-wavelength light, but they do not make the screen dark or remove the mental stimulation.

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Reducing brightness and putting the screen away may therefore have a larger effect than changing colour tone alone.

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Some people are more sensitive to light

Light does not affect everyone in the same way. Individual differences may relate to age, eye anatomy, chronotype, previous light exposure, medication, and genetic factors.

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Young eyes generally allow more light to reach the retina than older eyes. With age, the lens often becomes less transparent, and the circadian system may receive a weaker signal from the same environment.

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Older adults also tend to spend less time outdoors.

Morning types and evening types may also respond differently to the same timing. For a strong evening type, early morning light may be particularly helpful for shifting the rhythm earlier, although it may initially feel uncomfortable.

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People with migraine, certain eye conditions, or marked light sensitivity may need individual adjustments. More light is not always better if it triggers pain or other symptoms.

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How to use daylight more actively

It is rarely necessary to measure light levels or follow a complicated programme. For most people, the goal is simply to create a clearer difference between day and night.

Practical measures include:

• go outside relatively soon after waking

• eat breakfast or drink morning coffee near a window

• place a short walk in the morning or late morning

• take outdoor breaks rather than spending every break indoors

• move the workspace closer to daylight

• combine light exposure with gentle physical activity

• use adequate indoor lighting on dark days

• gradually dim the lights during the final hours before bed

• keep the bedroom dark at night

• wake at roughly the same time on most days

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The amount of time needed depends on light intensity. Bright outdoor light can provide a stronger signal in less time than weak winter light or indoor lighting.

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There is therefore no exact number of minutes that suits every person and every day.

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A practical solution is to make daylight part of an activity you already perform, such as commuting, walking the dog, taking phone calls, or eating lunch.

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Light therapy is stronger than ordinary indoor lighting

Light therapy lamps are used for conditions such as seasonal affective disorder and some circadian rhythm disturbances.

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They are designed to provide substantially more light than normal room lighting.

Timing is important. Morning light therapy may shift the circadian rhythm earlier, while exposure at another time can produce a different or even opposite effect.

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People who use medication that increases light sensitivity, have eye disease, bipolar disorder, or significant sleep problems should consider professional guidance before using systematic light therapy.

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Light therapy should not be confused with tanning beds. Tanning beds emit ultraviolet radiation and are not recommended for regulating circadian rhythm or energy.

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A proper light therapy device should be designed for the purpose and should filter out harmful ultraviolet radiation.

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Daylight does not solve every form of fatigue

Low energy can have many causes that are not corrected by more light.

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Daylight can support alertness and circadian timing, but it does not necessarily treat the underlying reason for persistent fatigue.

Other possible contributors include:

• insufficient or fragmented sleep

• sleep apnoea

• iron deficiency or anaemia

• an underactive thyroid

• infection or other illness

• depression

• prolonged stress

• insufficient energy intake

• medication side effects

• alcohol or other substances

• very high training volume

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The whole picture therefore matters.

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If you receive adequate daylight and still feel unusually exhausted over time, the solution should not be limited to more light or more caffeine.

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When persistent low energy should be assessed

It is normal to have occasional difficult mornings and periods of lower energy, particularly during winter, after poor sleep, or during periods of high stress.

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Medical assessment may be appropriate when fatigue persists, becomes progressively worse, or causes clear functional decline.

It may be sensible to contact a doctor if low energy occurs together with:

• marked low mood

• loss of interest or pleasure

• shortness of breath during light activity

• palpitations or dizziness

• unexplained weight change

• fever or night sweats

• severe sleep problems

• loud snoring and breathing pauses

• substantial muscle weakness

• concentration problems affecting daily life

• a need to sleep for large parts of the day

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Suicidal thoughts, acute confusion, chest pain, fainting, or severe breathing difficulties require urgent medical help.

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Summary

Daylight affects energy because light reaching the eyes tells the brain what time of day it is. Morning light helps end the biological night, can increase alertness, and helps place sleep, body temperature, and hormone activity at more appropriate times. Bright light during the day and dimmer light in the evening create a clearer difference between day and night. A morning walk or outdoor break may therefore make it easier to feel awake during the day and sleepy in the evening. Daylight cannot replace adequate sleep or treat every cause of persistent fatigue, but it is an important and practical part of circadian regulation.

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Sources

• Blume, C., Garbazza, C. & Spitschan, M. (2019). Effects of light on human circadian rhythms, sleep and mood. Somnologie, 23, 147–156.

• Tähkämö, L., Partonen, T. & Pesonen, A. K. (2019). Systematic review of light exposure impact on human circadian rhythm. Chronobiology International, 36(2), 151–170.

• Siraji, M. A., Kalavally, V., Schaefer, A. & Haque, S. (2022). Effects of daytime electric light exposure on human alertness and higher cognitive functions: A systematic review. Frontiers in Psychology, 12, 765750.

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