For almost all of human history, food has been a scarce resource, and periods of fasting—even long ones—were fairly common. For many of us today, however, our next meal is no farther away than the kitchen pantry. Overconsumption of food, not underconsumption, is more likely to be our biggest health threat.
In recent years, though, many people have been experimenting with intermittent fasting (IF) as a tool for weight loss and longevity. IF means designating regular times to stop eating, usually specific parts of every 24-hour cycle, based on the theory that our ancient genes are expecting plenty of time away from food to quiet the digestive processes and let the body repair and restore itself. IF has been touted as a tool for weight loss, disease prevention and longevity, and there’s good evidence that it works.1
Food and the circadian rhythm
So what does this have to do with sleep? Unless you’re regularly sleep-walking straight to the refrigerator, sleep happens to be a built-in period for intermittent fasting—ideally eight or nine hours of it. And there’s evidence that food consumption is part of the body’s natural circadian rhythm,2 the 24-hour internal “clock” that slows us down when it’s time to sleep and revs us up during the day.
Ensuring that we’re not eating at night3 can strengthen our circadian rhythm, reinforcing the pattern of a bright, warm, active and alert body during the day and a dark, cool, quiet and sleepy one at night. And studies show that extending the natural fasting state that occurs during eight or nine hours of sleep into a 10- to 12-hour fasting period has other benefits4 as well, including improving the quality of sleep, reducing the number of awakenings, and calming leg movements during sleep.
If you’re thinking about trying IF for its sleep and other health benefits, you should be aware of a couple of caveats.
First, understand that fasting is a form of stress5 on the body—just as exercise is—which means it can increase cortisol and noradrenaline levels. These are hormones that affect your circadian rhythm and can keep you awake. So just like with an exercise routine, it’s a good idea to start small and listen to your body. For example, if you’re currently eating for 16 hours a day, start by cutting off your food intake an hour earlier or waiting an additional hour before breakfast. If you don’t notice any detriment to your sleep, gradually increase the time without food up to 10 to 12 hours, say from 7 p.m. to 7 a.m. (And if that seems to suit you, many people swear by a 16-hour fasting period every day with an eight-hour “feeding window,” for example from 10 a.m. to 6 p.m.)
Second, be careful about cutting calories too much—or even at all. The goal is to eat an appropriate amount of food during the time you’re not fasting, so your meals will need to be fewer but larger. Cutting calories too low is another stressor for the body, and undereating has been linked to poor-quality sleep.6 Remember to stay hydrated, too: Beverages without calories, such as water, tea and coffee, are fine during your fasting hours.
Finally, remember that sleep and nutrition needs are individual. Just as you might need to tweak your sleep and wake times to find what works for you, you may also need to see what eating and fasting patterns are best. People with blood sugar issues might find that fasting doesn’t help them at all. For many, though, IF can be an easy tweak that will pay dividends in the form of weight loss, better health and sounder sleep.
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- “Circadian Code to Extend Longevity,” talk by Satchin Panda, TEDxVeniceBeach, November 2018. https://www.youtube.com/watch?v=wrP78K1objc
- “Circadian Rhythm and Sleep,” American Sleep Association. https://www.sleepassociation.org/sleep-disorders/circadian-rhythm/
- “Metabolism and the Circadian Clock Converge,” Physiological Reviews, January 2013. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3781773/
- “Can Intermittent Fasting Help Sleep?” by Dr. Michael Breus, Sleep Doctor, December 2018. https://thesleepdoctor.com/2018/12/18/can-intermittent-fasting-help-sleep/
- “The Dangers of Intermittent Fasting,” by Liz Meszaros, MDLinx. https://www.mdlinx.com/article/the-dangers-of-intermittent-fasting/6CZfO5sYFDWi4wK2OPRiEd/
- “An Examination of the Association Between Eating Problems, Negative Mood, Weight and Sleeping Quality in Young Women and Men,” Eating and Weight Disorders, December 2005. https://pubmed.ncbi.nlm.nih.gov/16755168/