When people struggle to sleep, they usually concentrate on what happens immediately before bedtime. They may avoid caffeine, turn off the television, change their mattress or try to make the bedroom quieter.

Those measures can be helpful, but new research suggests that an important part of healthy sleep begins much earlier: the amount and regularity of light received during the day.

Researchers from the University of Manchester found that adults who received brighter, more consistent daytime light generally went to sleep earlier, awakened earlier and experienced more deep sleep during the first portion of the night. The findings were published in the scientific journal npj Biological Timing and Sleep. You can read the complete study here: Light Exposure and Sleep Architecture in Real-World Settings.

What the researchers studied

The study included 89 adults in the United Kingdom. Participants simultaneously wore personal light sensors and consumer-grade sleep trackers for seven days and completed daily sleep diaries.

Altogether, the researchers collected more than 500 person-days of light and sleep information. Light exposure was evaluated using a measurement called melanopic equivalent daylight illuminance, which estimates how strongly light stimulates the eye’s circadian light-sensing system.

The researchers found that:

• Longer exposure to brighter daytime light was associated with earlier sleep and wake times.
• More stable light exposure from day to day was associated with healthier sleep timing.
• People with more regular light patterns experienced stronger deep sleep during the early portion of the night.
• Irregular swings between dim and bright environments were associated with less favorable sleep patterns.

The University of Manchester’s summary of the findings emphasizes that both brightness and regularity appeared important.

How light communicates with the brain

Light does more than allow us to see. Specialized light-sensitive cells in the retina communicate with the brain’s central circadian clock.

This internal clock helps coordinate:

• daytime alertness;
• evening sleepiness;
• melatonin production;
• body temperature;
• hormone rhythms;
• digestion and metabolism;
• and the timing of sleep stages.

Bright light during the morning and daytime tells the nervous system that it is time to be awake and active. As daylight decreases and the environment becomes darker, the body receives a contrasting signal that nighttime is approaching.

The body therefore benefits from a clearly defined rhythm:

Bright days, dim evenings and dark nights.

Modern indoor living can reverse that natural pattern. A person may spend most of the day in a relatively dim office and then receive strong artificial light from overhead fixtures, televisions, phones and computer screens late in the evening.

Why morning light can be especially helpful

Morning light generally helps anchor the beginning of the biological day. Regular early-day light exposure can support a more consistent circadian rhythm and may help evening sleepiness occur at a more appropriate time.

The new study did not establish a universal number of minutes that everyone must spend outside. Its main finding was that people receiving brighter and more consistent daytime light had more favorable sleep patterns.

Outdoor daylight is often substantially brighter than ordinary indoor lighting, even when the sky is cloudy. A person does not necessarily need to stare toward the sun. Simply being outdoors and allowing normal ambient daylight to reach the eyes can provide a much stronger circadian signal than remaining inside a dim room.

Directly looking at the sun should be avoided.

Evening light may send the opposite signal

Light received later in the day can have a different effect from morning light. Depending on its intensity, duration and timing, bright evening light may increase alertness and interfere with the normal evening rise in melatonin.

A controlled study published in the same journal found that bright afternoon-to-early-evening light had acute alerting effects and reduced evening melatonin under the tested conditions. Read the research here: Afternoon-to-Early-Evening Bright Light Exposure and Melatonin.

This does not mean that every lamp or brief glance at a telephone will inevitably cause insomnia. The effect depends on several factors, including:

• brightness;
• distance from the eyes;
• duration of exposure;
• time of day;
• previous light exposure;
• and the individual’s circadian sensitivity.

Nevertheless, reducing unnecessarily bright light during the last one or two hours before bedtime may help reinforce the transition from daytime alertness to nighttime rest.

A practical daytime-light routine

Shortly after awakening

Open the curtains or blinds and obtain outdoor light as early as practical. A short walk, sitting on a porch or completing an outdoor task can combine daylight exposure with gentle physical activity.

During the morning

Avoid spending the entire morning in a dim room when brighter light is available. Work near a window where practical, although outdoor light generally produces a stronger signal than light filtered through a building.

Throughout the day

Take periodic outdoor breaks. The goal is not excessive sun exposure but a healthy distinction between the brightness of the day and the relative darkness of the evening.

During the evening

Reduce unnecessarily bright overhead lighting. Use sufficient light for safety and reading, but consider gradually dimming the environment as bedtime approaches.

During sleep

Keep the bedroom as dark as reasonably possible. Light from televisions, hallway fixtures, illuminated clocks and other devices can weaken the normal nighttime signal.

Maintain a regular wake time

Try to awaken at approximately the same time each day. Large differences between weekday and weekend schedules can create a form of “social jet lag” and make sleep timing less predictable.

The study’s important limitations

The results are encouraging, but the study should not be presented as proof that increased daylight will cure a sleep disorder.

This was an observational study, meaning researchers measured naturally occurring light exposure and sleep rather than randomly assigning participants to controlled light treatments. It can therefore identify associations but cannot establish with certainty that brighter light directly caused the sleep differences.

Additional limitations included:

• a relatively small group of 89 adults;
• approximately seven days of monitoring per participant;
• light measured at the wrist rather than directly at the eyes;
• sleep stages estimated by consumer wearables rather than laboratory polysomnography;
• and possible influences from exercise, meals, work schedules, health conditions and time spent outdoors.

It is possible, for example, that people who spent more time outside were also more physically active. That activity could have contributed to some of the observed sleep benefits.

A responsible conclusion is therefore:

Brighter and more consistent daytime light was associated with earlier sleep timing and stronger early-night deep sleep, but controlled trials are needed to determine how much improvement is directly caused by changing light exposure.

Who may benefit most from improving daytime light exposure?

Attention to daily light patterns may be especially worthwhile for people who:

• work indoors for most of the day;
• receive little morning light;
• feel sleepy during the day but alert late at night;
• have an irregular bedtime or wake time;
• use bright lighting and electronic devices late at night;
• experience seasonal sleep changes;
• or have difficulty adjusting their sleep schedule.

Daytime light management may also complement nutritional support, regular exercise, stress reduction and improvements to the bedroom environment.

Light exposure is not a substitute for evaluating persistent sleep problems

Daylight exposure may strengthen circadian timing, but it will not correct every cause of disturbed sleep.

Persistent sleep difficulty may also involve:

• obstructive sleep apnea;
• chronic pain;
• restless legs;
• blood-sugar instability;
• thyroid or hormonal disturbances;
• medication effects;
• anxiety or excessive mental stimulation;
• alcohol use;
• breathing difficulties;
• or an inconsistent work schedule.

People who regularly awaken gasping, snore heavily, experience marked daytime sleepiness or remain unable to sleep despite improving their routine should receive an appropriate evaluation.

The practical conclusion

Sleep is not controlled only by what happens at bedtime. The brain continually uses environmental light to determine whether it should prepare the body for wakefulness or sleep.

A healthier daily pattern may be summarized simply:

Seek brighter, more consistent light during the morning and daytime. Reduce excessive brightness in the evening. Sleep in darkness and maintain a regular wake time.

This approach is inexpensive, natural and compatible with other measures intended to improve sleep quality. The recent study does not prove that daylight alone will transform sleep, but it provides meaningful evidence that the quality of the night may be influenced by the brightness of the day.

Sources

• Read the complete University of Manchester study in npj Biological Timing and Sleep
• Read the University of Manchester explanation of the findings
• Read the study on afternoon and early-evening bright light, alertness and melatonin