Clocks Go Back November 1: The Surprising Link Between Daylight and BMI

On Sunday, November 1, daylight saving time ends across most of the United States. Clocks move back an hour, bringing earlier sunrises and darker evenings. In New York City, for example, the sun will rise around 6:27 a.m. on Monday, November 2, giving early commuters another chance to enjoy some morning daylight.

Most people focus on the extra hour of sleep or how quickly darkness arrives after work. But sleep researchers are particularly interested in what happens during the morning.

What changes when daylight saving time ends?

At 2 a.m. on November 1, clocks move back to 1 a.m. in most states. Hawaii and most of Arizona don't observe daylight saving time.

Sunrise and sunset will occur approximately one hour earlier by the clock. The actual times vary considerably depending on where you live.

This temporary return to brighter mornings could be particularly useful for Americans who leave for work early and spend much of their day indoors.

However, the advantage gradually disappears as sunrise gets later toward December.

What daylight does for your sleep

A 2021 study involving more than 400,000 UK Biobank participants examined how time spent outdoors during daylight relates to sleep and mental wellbeing.

Participants reported spending a median of approximately 2.5 hours outside each day. Researchers found that each additional hour outdoors was associated with fewer insomnia symptoms, less tiredness, and greater ease getting out of bed.

The explanation involves your circadian rhythm, the internal body clock that helps regulate when you feel alert and when you become sleepy.

Morning daylight provides an important signal that helps synchronize this rhythm with the 24-hour day.

However, the British study was observational. It cannot establish that extra outdoor time directly caused the improvements.

A Northwestern study found something unexpected

Researchers at Northwestern University's Feinberg School of Medicine investigated whether the timing of light exposure might also be associated with body weight.

In their 2014 study, 54 adults wore wrist monitors for one week to measure their exposure to light and their sleep patterns. Participants also kept food diaries.

Those receiving brighter light earlier in the day generally had lower BMIs than participants exposed to similar light later.

The relationship remained after researchers accounted for physical activity, calorie intake, sleep patterns, age and season.

The timing of light exposure explained approximately 20 percent of the variation in BMI between participants after adjustments.

Researchers highlighted light levels around 500 lux and suggested that 20 to 30 minutes of bright morning light could be a useful target.

But there's an important limitation: this small study identified an association, not proof that morning sunlight causes weight loss.

Why outdoor light makes a difference

Even on overcast mornings, outdoor daylight is usually much brighter than typical indoor lighting.

The Northwestern researchers noted that many workplaces provide just 200 to 300 lux of light, while cloudy outdoor conditions can exceed 1,000 lux.

That difference is important because the body's internal clock responds to both the intensity and the timing of light.

Sitting in a brightly lit kitchen or office may therefore provide substantially less light than spending time outdoors.

The morning habit worth trying this November

You don't need to change your entire schedule.

Try taking a 20- to 30-minute walk after sunrise, having your morning coffee on the porch, or walking the dog before starting work. If possible, aim for outdoor daylight during the first few hours after waking.

Researchers particularly highlighted the period between 8 a.m. and noon. And if mornings aren't possible, getting outside later is still worthwhile. The evidence for daylight's relationship with sleep is broader than the evidence linking morning light to BMI.

With the clocks going back, the coming weeks offer a convenient opportunity to spend a little more time outdoors before your day gets busy.