Sleep debt explained: how it builds up and how to recover

Sleep debt is the gap between the sleep you need and the sleep you actually get, added up over days or weeks. Sleeping an hour short each weeknight builds a debt of five hours by Friday. You can recover from a short-term debt with a few nights of extra sleep, but weekend lie-ins only partly repair a pattern of short nights. The sleep debt calculator adds up a week and suggests a recovery plan; this guide explains the idea and its limits.

What sleep debt is

Researchers distinguish between total sleep deprivation, staying awake for 24 hours or more, and partial sleep deprivation, sleeping too little for several nights or weeks. Sleep debt usually means the second: the common pattern of late nights, early alarms and a long lie-in at the weekend. Sleep debt is not a medical diagnosis, and scientists still debate exactly how it accumulates and how it is repaid, but the basic idea, that missed sleep adds up and shows in how you function, is well supported.

How to work it out

Start with your sleep need. Most adults need 7 to 9 hours, teenagers 8 to 10 and adults over 65 about 7 to 8; How much sleep do you need? Recommended hours by age explains how to find your own. Then record the hours you actually slept each night, not the time in bed, and subtract each night from your need. The calculator does this for the last seven nights. Its example week is 6.5, 6, 7, 5.5, 6.5, 9 and 8.5 hours against an 8-hour need: 49 hours slept instead of 56, an average of 7 hours a night and a debt of 7 hours. Note that the long weekend nights reduce the total but do not cancel it. If your nightly average drops below 6 hours, the calculator flags it, since that level is linked to poorer health, mood and concentration.

What it does to you

Short-term sleep debt shows up as tiredness, irritability, poorer concentration and slower reactions. In one well-known laboratory study, people restricted to 6 hours a night performed progressively worse on an attention test, and after about 10 days their results were similar to people kept awake for a whole night. Short sleep also affects emotions, making people react more strongly to negative events, and appetite hormones, which may help explain the link between short sleep and weight gain. Over years, regularly sleeping too little is associated with higher risks of conditions such as type 2 diabetes, high blood pressure and heart disease, as well as depression.

Can you catch up?

Partly. A few nights of extra sleep relieve the tiredness and much of the short-term impairment from a few bad nights. Some studies have also found that longer weekend sleep is associated with lower health risks among people who sleep short on weekdays, compared with people who sleep short every day. But other research suggests that recovery sleep does not fully reverse every effect of a long-running debt, particularly on metabolism, and big weekend lie-ins shift your body clock later, making Sunday night and Monday morning harder. Catching up is a useful repair, not a substitute for enough sleep most nights.

How to pay it back

• Add a modest amount each night. The calculator suggests 30 minutes, 45 minutes or an hour extra per night and shows how many nights that takes; at an extra hour, the 7-hour example debt clears in about a week.
• Go to bed earlier rather than sleeping in, which keeps your wake-up time and body clock steady.
• Use short naps carefully. A brief early-afternoon nap can help, but long or late naps can make it harder to sleep at night.
• Cut late caffeine. The caffeine calculator shows how much is still in your system at bedtime.
• Time your night. The sleep calculator suggests bedtimes based on 90-minute cycles for a fixed wake-up time.

After the debt is cleared, a steady routine at your normal sleep need matters more than any catch-up plan. If you want to see how sleep affects your days, tagging sleep in the mood journal shows your average mood on good- and poor-sleep days.

When it is more than debt

Sometimes the problem is not too few hours in bed. See a GP if you regularly struggle to fall or stay asleep despite having time to sleep, especially for three months or more; if you snore loudly, gasp or choke at night, or someone notices your breathing stop and start, which can be signs of sleep apnoea; or if you sleep a full night and still wake exhausted or fall asleep during the day. Never drive or operate machinery when you feel very sleepy.