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Students' sleep patterns were studied by a team of researchers led by the University of Washington (UW), who discovered that during the winter, when there are less daylight hours and typically gloomy skies, participants slept later at night and woke up later in the morning. The reason why this phenomena occurs in the winter is because humans are exposed to less light during the day.

Senior author and UW professor of biology Horacio de la Iglesia stated, "Our bodies have a natural circadian clock that informs us when to go to sleep at night. Your clock "delays" and "pushes back" the start of sleep at night if you do not get adequate exposure to light during the day when the sun is out.

From 2015 to 2018, the researchers employed wrist trackers to look at the sleep habits and light exposure of 507 UW undergraduate students. While the analysis revealed that students were sleeping roughly the same amount each night regardless of the season, it was rather surprising to learn that during the winter months, students were sleeping an average of 35 minutes later and waking up an average of 27 minutes later than they were during the summer.

Due of the abundance of light throughout the summer, de la Iglesia explained, "we were anticipating that students would be up later during that period." The pupils' circadian cycles, however, seemed to be "pushed back" throughout the winter. The natural circadian cycle that determines when most people are awake and asleep lasts roughly 24 hours and 20 minutes and is adjusted by environmental factors. The results of this study showed that the individuals' circadian rhythms were almost 40 minutes later in the winter than in the summer.

Despite the fact that light seems to be a significant factor in this phenomena, the researchers emphasised that light affects circadian rhythms differently depending on the time of day. "Light exposure late in the day or early in the night can delay your clock, putting back the time that you will feel exhausted," explained de la Iglesia. "Light exposure during the day — especially in the morning — advances your clock, so you get sleepy sooner in the evening." The push and pull between these opposing impacts of light exposure at various times of the day ultimately determines when you fall asleep.

The researchers found that each hour of artificial nighttime light delayed circadian phases by an average of 15 minutes, whereas each hour of midday light advanced participants' circadian phases by 30 minutes.

Many of us live in urban areas with lots of artificial lighting and indoor-oriented lifestyles. This study demonstrates the need of getting outside, even if only briefly and especially first thing in the morning, to receive natural light. Reduce screen time and artificial lighting in the evening to promote sleep, according to de la Iglesia.

In the Journal of Pineal Research, the work has been published.