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You can't repay a sleep debt
Most of modern society cuts sleep short during the week and binges on the weekend. That pattern is called sleep bulimia, or social jetlag. Take someone, deprive them of one night of sleep, eight hours lost, and then let them sleep as much as they want for as many nights as they like. The brain will try to get some of those hours back, and it will never get back all of them.
Biology does have credit systems elsewhere. A fat cell stores calories as credit and spends them in a famine, which evolution built because humans faced famine.
Sleep has no equivalent because humans are the only species that deliberately deprives itself of sleep for no apparent reason. Evolution never had to produce a safety net for insufficient sleep, so there is no mechanism of safety and no credit system. Different parts of the brain and body also return to health at different speeds with recovery sleep, and some components of the immune system take longer than others.
How does a fat cell store energy?
Fat cells, called adipocytes, package spare calories as triglycerides, molecules of fatty acids bound to glycerol. When blood sugar and insulin fall during fasting, hormones signal the cells to break the triglycerides down and release fatty acids into the blood. Humans carry enough stored fat to supply energy for weeks. Fat tissue is the body's largest energy reserve.
Does weekend catch-up sleep fix a short week?
Researchers at the University of Colorado Boulder tested this in a 2019 study published in Current Biology. Healthy adults were held to five hours of sleep a night for five days, and one group was then allowed to sleep in on the weekend. The weekend sleepers still showed reduced insulin sensitivity, and then lost it again when short sleep resumed. They also snacked more after dinner.
What happens in the brain during recovery sleep?
After a night without sleep, the brain does not repay the lost hours evenly. Sleep scientists have long observed a rebound in deep slow-wave sleep first, with a rebound in REM sleep coming later. The recovery night is therefore shorter and denser than the hours lost. William Dement, a founder of sleep medicine, documented rebound effects like these in the 1960s.
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