What is sleep debt: six hours a night can feel fine, but your body disagrees

Many people sleep six hours a night for weeks at a time and feel they are still working normally. Sleep research suggests that feeling may not match what is actually happening.

How much sleep adults need

A joint consensus statement from the American Academy of Sleep Medicine and the Sleep Research Society (Watson et al., Sleep 2015) recommends that adults sleep at least 7 hours a night on a regular basis. It states that regularly sleeping less than 7 hours is linked to a range of poor health outcomes, including weight gain, diabetes, high blood pressure, heart disease, depression, weaker immune function, and more errors and accidents.

What you miss each night, compared with what your body needs, adds up into what is called sleep debt.

Short sleep, night after night: steadily worse, without noticing

In an experiment by Van Dongen et al. (Sleep, 2003), 48 healthy adults were allowed 4, 6 or 8 hours of sleep a night for 14 days in a row, and were compared with a group kept awake for three full nights. The results:

• The 4-hour and 6-hour groups got steadily worse at thinking and staying alert as the days went on.
• The researchers concluded that sleeping 6 hours or less night after night could bring test performance down to the same level as two full nights without sleep.
• But the sleepiness participants reported rose only at the start, then barely rose further. The researchers described participants as "largely unaware" that their performance kept getting worse.

Can you catch up on weekends

Depner et al. (Current Biology, 2019) studied 36 adults split into three groups: one slept 9 hours, one slept 5 hours throughout, and one slept 5 hours on weekdays and as much as they wanted at the weekend. They found that:

• Over the weekend, the catch-up group slept only about 1.1 hours more than usual in total.
• When they went back to short sleep after the weekend, insulin sensitivity across the whole body, the liver and the muscles fell by about 9–27% compared with the start.
• The researchers concluded that weekend catch-up sleep is not an effective way to prevent the metabolic harm of repeated short sleep.

What actually helps

1. Get enough sleep, regularly. The recommended goal is at least 7 hours a night, every night, not an average propped up by the weekend. 2. Look at your sleep as a trend across the week. One short night isn't a big problem, but several in a row add up. 3. Don't rely on how sleepy you feel alone, because the research above found we may not notice. 4. If you regularly can't sleep or feel very sleepy during the day, talk to a doctor.

The Health tab in Align shows your sleep hours every morning when you connect a supported wearable. You can read more about sleep stages in the previous article.

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*This article is general information, not medical advice. If you have sleep problems or health concerns, please talk to a doctor.*

Sources

• Watson NF, et al. Recommended Amount of Sleep for a Healthy Adult: A Joint Consensus Statement of the American Academy of Sleep Medicine and Sleep Research Society. Sleep 2015;38(6):843–844. https://pmc.ncbi.nlm.nih.gov/articles/PMC4434546/
• Van Dongen HPA, et al. The cumulative cost of additional wakefulness: dose-response effects on neurobehavioral functions and sleep physiology from chronic sleep restriction and total sleep deprivation. Sleep 2003;26(2):117–126. PMID 12683469
• Depner CM, et al. Ad libitum Weekend Recovery Sleep Fails to Prevent Metabolic Dysregulation during a Repeating Pattern of Insufficient Sleep and Weekend Recovery Sleep. Current Biology 2019;29(6):957–967. doi:10.1016/j.cub.2019.01.069