Daylight exposure helps ease better into sleep at night, study suggests


Getting insufficient sunlight exposure during the day may lead to problems sleeping at night, according to a new study which recommends that people go out in the morning sun, even if it is for a “little while.”

The research, published last week in the Journal of Pineal Research, used wrist monitors to follow the sleep and light exposure patterns of over 500 undergraduate students at the University of Washington (UW) in the US from 2015 to 2018.

Scientists found that the students fell asleep later in the evening and woke up later in the morning mostly during winter when their hours of exposure to daylight on the campus are limited.

“Our bodies have a natural circadian clock that tells us when to go to sleep at night. If you do not get enough exposure to light during the day when the sun is out, that ‘delays’ your clock and pushes back the onset of sleep at night,” study senior author Horacio de la Iglesia, a professor of biology at UW, said in a statement.

Scientists found that while students were getting roughly the same amount of sleep each night regardless of season, in winter, they were going to bed on average 35 minutes later and waking up 27 minutes later than summer.

“We were expecting that in the summer students would be up later due to all the light that’s available during that season,” Dr de la Iglesia said.

Researchers suspect something may be “pushing back” the students’ circadian cycles in winter.

They say the body’s internal clock which governs when people are awake and asleep runs at about 24 hours and 20 minutes, “calibrated” daily by input from our environment.

However, the study suggests these circadian cycles for the UW students were running up to 40 minutes later in winter compared to summer.

“Light during the day — especially in the morning — advances your clock, so you get tired earlier in the evening, but light exposure late in the day or early night will delay your clock, pushing back the time that you will feel tired,” Dr de la Iglesia said.

“Ultimately, the time that you fall asleep is a result of the push and pull between these opposite effects of light exposure at different times of the day,” he added.

The new research also suggests daytime light exposure may have greater impact than evening light exposure.

It found that each hour of daytime light “moved up” circadian phases of the students by about 30 minutes.

While every hour of evening light, such as those from indoor sources like lamps, delayed the biological clock’s phases by an average of 15 minutes, scientists say.

“It’s that push-and-pull effect. And what we found here is that since students weren’t getting enough daytime light exposure in the winter, their circadian clocks were delayed compared to summer,” Dr de la Iglesia explained.

With a growing number of people across the world moving to live in cities and towns with artificial light and lifestyles keeping them indoors during the day, researchers call for these city-dwellers to head out, “even for a little while and especially in the morning,” to get natural light exposure.

“In the evening, minimize screen time and artificial lighting to help us fall asleep,” Dr de la Iglesia added.

“Our results show that although sleep time is highly synchronized to social time, a delayed timing of sleep is evident during the winter months. They also suggest that daily exposure to daylight is key to prevent this delayed phase of the circadian clock,” researchers wrote in the study.



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