Light has a profound effect on our sleep and waking. Humans are diurnal - we evolved to sleep at night. It is easier to sleep in the dark than in light, despite what some people may claim about their personal habits.
External environmental and social cues set the phase for the circadian cycle. The most powerful cue is daylight. Sunrise, high noon, and sunset are milestones in nature’s clock - a clock that encompasses and influences plants, animals, and human society. The ganglion cells in the eyes’ retinas are connected directly to the suprachiasmatic nucleus (SCN), the "master clock" for the body. Indeed, the discovery ofnew kind of light-detecting retinal cells - "intrinsically photosensitive retinal ganglion cells" (ipRGCs) - (in addition to the rods and cones you learned about in high school) is part of the uncovering of the mysteries of sleep in recent decades.
Light is so important in establishing the circadian time that it is safe to say that it swamps all other signals. When people get jet lag, the "cure" is entrainment to the local time, which can be effected by spending time outdoors.
Light exposure can have profound effects on sleep. When the environment transitions from darkness to light, our brains tend to wake, whether we have had our regular sleep or not. It is harder to sleep in bright light than dim light. But exposure to bright light during the day also makes sleep feel deeper and more satisfying the next night.
Low light is thought to be a cause of sleep disorders in people who are institutionalized and spend all day inside. It is suggested that bright light exposure can improve sleep quality in the population of Alzheimer’s disease communities.
Light therapy is used for people with circadian disorders; this light therapy can be in the form of artificial light (light tables). Although experts agree outdoor sunlight is more effective than light tables, the artificial light (if bright enough) can be useful when needed.
Light therapy in combination with sleep restriction has been shown to be an effective way of raising mood in depressed people, although it is not widely used and the sleep restriction part would make it unsustainable over more than a few days.
People perform better on cognitive and physical tasks when it is light than when it is dark, independent of whether they feel sleepy. It is no coincidence that people describe an upbeat mood as “sunny” or good news is referred to as “brining a little sunshine in”. Bright list affects our moods. And our sleep quality. A study of people working in office buildings found those in windowless environments tended to have lower scores on sleep quality tests than those who got to see the Sun during the workday.
The military has determined that early morning light is the best way to keep pilots awake, and flight headgear is sometimes fitted light designed to strike the eye from the side and in a color with a blue-ish tint.
Quick cure for insomnia: going out during the day. That’s a little simplistic, but here is the idea. The contrast between a brightly lit part of the day and the dark night is important in telling the brain that it is time to sleep at night. Even brightly lit interiors such as office buildings and health clubs are dim compared to the outdoors. Brightly lit indoors might be 100 Lux, while outdoors on an overcast day is 10,000 Lux. You may not consciously register how different the light levels are because our eyes adjust to the environment, but even overcast days during the winter are bright compared to indoors.
The mid-day light exposure also increases release of melatonin at night. The circadian phase doesn’t shift. So science has established a biochemical basis for the claim that being outdoors during the day can help you fall asleep at night.
There is also evidence from brain imaging that the waking brain is more active when the ambient light is bright. Even if we are not conscious of it and even if we find bright light distracting, we are more mentally nimble in bright light. Folk wisdom has long held that sunshine makes you happy, and indeed formal psychological testing has found positive effects from daylight.
People in Antarctica in the winter (no significant sunlight) often get delayed sleep phase syndrome or free-running disorder. Bright artificial light tinged with blue has been shown to help afflicted individuals get back on track, and the light therapy does not cause health problems.
An hour a day of bright light has been shown to improve cognitive function and work task performance. Other physiological markers are affected by the regular exposure to bright light. It is a technological fix for an unusual situation – we did not evolve in the arctic – but it works.
Astronauts in an even more artificial environment (a space station or shuttle) also have sleep problems, due to low lighting and to other artificial conditions including low gravity. The managers of the space program allow sufficient time for sleep and endeavor to provide exercise opportunities for people on the station. But even with these fixes, astronauts on long missions often have poor sleep. Given the difficulty of the jobs on the space station, this has raised ergonomic concerns and human factors engineering work by NASA.
National Geographic: How Light Wakes Up The Brain