Do sleeping hours count toward a fast?

The hours count while you sleep

The timer stores the time you stopped eating and subtracts it from the current time. If the last meal was at 8 p.m. and the next meal is at noon, the screen shows those hours whether you woke at 6 or slept until noon. There is one count, the time since you stopped eating.

Names such as Fat Burning or Ketosis appear because those hours have passed. The hour ranges, and what the names are for, are in What the stages on the timer mean.

If you have not chosen a first window, How to start fasting starts from the overnight gap you already have, after dinner until breakfast.

Energy use is lower during sleep

In 2011, Christopher M. Jung, Edward L. Melanson, Emily J. Frydendall, Leigh Perreault, Robert H. Eckel, and Kenneth P. Wright measured energy use in 7 healthy adults, 5 men and 2 women, average age about 22. They published the results in The Journal of Physiology. The people lived in a room that measured the energy they used. One night they slept about 8 hours at their usual time. Another night they stayed awake for those same hours, resting in bed, propped up, in dim light. Meals were set to match the calories needed to hold their weight. The people were still eating.

During those 8 night hours, energy use was about 32 percent higher while they were awake than while they slept, about 161 kilocalories. Across the whole day, missing that night of sleep raised energy use by about 7 percent, about 134 kilocalories. The authors call the drop during the night relatively large, and they say the savings across the whole day appears small. They also say the higher energy use of a missed night should not be read as a safe or effective way to lose weight. They note that other studies have tied ongoing sleep loss to worse thinking and to weight gain.

The authors write, “It is likely that the metabolic cost of sleep deprivation in free-living individuals would have been even greater due to activity-associated energy demands (e.g. ambulation).” The study did not measure someone up and moving. The numbers above are for those 8 hours resting in bed, propped up.

Hunger is something you notice while you are awake

In 2013, Frank A. J. L. Scheer, Christopher J. Morris, and Steven A. Shea studied 12 healthy adults, 6 men, ages 20 to 42, with a body mass index from 19.9 to 29.6. They published the results in Obesity. For 13 days in a laboratory, meals and sleep were scheduled so that the time of day on the body clock could be separated from how long someone had been awake. People rated hunger on a line while they were awake. One person was left out of that analysis because they used the line incorrectly. The people were still eating.

Hunger was lowest around 8 a.m. on that internal clock and highest around 8 p.m. The gap from the low to the high was 17 percent of the average rating. The pattern did not depend on how long it had been since waking, or since the last meal. In people who sleep at night, the authors say the evening peak may lead to a larger meal before the fast that sleep already requires, and that a fall in hunger across the night would in theory work against a rise that could wake someone. They took the ratings only during scheduled wake. They did not measure hunger during sleep.

Read against that rhythm, 6 a.m. to noon falls nearer the low point than the high. The study did not test a long fast. Sleeping until noon means you are not awake to notice hunger during those morning hours. The timer still shows the hours since the last meal. The authors say people with obesity, diabetes, or shift work may differ.

A common 16:8 window runs through the night

In 2020, Dylan A. Lowe, Nancy Wu, Linnea Rohdin-Bibby, and colleagues ran a 12-week trial in JAMA Internal Medicine. They enrolled 116 adults, ages 18 to 64, with a body mass index from 27 to 43. The time-restricted group was told to eat as they liked from noon to 8 p.m. and to take no calories from 8 p.m. until noon the next day. Only drinks without calories were allowed outside that window. The fasting stretch they assigned includes the night and the next morning.

Weight fell by about 0.94 kilograms in that group and by about 0.68 kilograms in the group told to eat three meals a day. The gap between the groups could have been chance.

What remains open

This note found no study that compared the same fasting hours spent asleep and awake. In the energy study and the hunger study, the people were still eating their scheduled meals.

Do not stay in bed longer so that more of the fast happens while you are asleep. The hour count stays the time since the last meal. If sleep is already short, shorten the fast. That adjustment is in Sleep, stress, and your fasting window.