Why do I get ill easily after several nights of short sleep?

Several short nights in a row add up to a sleep debt, and research links regularly sleeping less than 6 to 7 hours with a higher chance of getting ill after coming into contact with a virus. What you can do is aim for 7 hours or more again, get back to regular sleep times rather than relying on naps or one long lie-in, and wash your hands often when you spend time in crowded places.

Why do several nights of short sleep make me get ill more easily?

One late night rarely feels like a problem. The trouble is that short nights tend to come in a row: a deadline week, a run of early meetings, exam season, a baby who wakes at night, or simply scrolling on your phone until late every night.

Short nights add up to a sleep debt

The US National Heart, Lung, and Blood Institute (NHLBI) explains that if you lose 2 hours of sleep each night, you will have a sleep debt of 14 hours after a week. Going to bed at 1 a.m. instead of 11 p.m. for five working days already adds up to 10 hours. The NHLBI also points out that a common myth is that people can learn to get by on little sleep with no negative effects.

What studies of sleep and getting ill have found

Two studies from the United States looked at how long people slept before they were deliberately given a virus through nasal drops, and then followed who became ill over the next 5 days.

Both studies measured sleep over a week or more, not a single night, which is why they speak to the pattern of several short nights in a row. In the 2015 study, people sleeping 6 to 7 hours were not clearly different from those sleeping more than 7 hours. Both were small studies, and the ranges around the results are wide, so they show a link rather than an exact amount of risk.

Sleeping lightly counts too

The 2009 study also looked at how much of the time in bed people actually spent asleep. Those asleep for less than 92% of their time in bed were 5.50 times more likely to become ill than those asleep for 98% or more. In other words, both the number of hours and how settled your sleep is seem to matter.

The US CDC lists getting sick less often among the benefits of getting enough sleep, alongside lower stress.

For your skin after several nights of short sleep, see Short sleep for days and my skin looks bad. What's missing?

How many hours of sleep is enough?

The CDC recommends the following:

Signs you may not be getting enough

The NHLBI says that sleeping a lot more on days off than on workdays can be a sign that you are not getting enough sleep. If you sleep until noon every Sunday after a week of 6-hour nights, your body is likely carrying a debt from the week.

Count back from your wake-up time

If you have to leave home at 7 a.m. and need an hour to get ready, you wake at 6 a.m. For 7 hours of sleep, you need to be asleep by 11 p.m., which means getting into bed a little earlier than that. Writing the time down makes it easier to protect.

I've been short on sleep for several nights. How do I get back on track?

Return to regular nights rather than one long lie-in

The NHLBI advises keeping the same sleep schedule on weeknights and weekends, and limiting the difference to no more than about an hour. So instead of sleeping until noon on Saturday, go to bed earlier for the next few nights and keep your usual wake-up time.

Keep naps short

Naps may make you more alert for a short time, but the NHLBI notes that they do not supply all the other benefits of night-time sleep, so you cannot really make up for lost sleep with naps. It advises adults to nap for no more than 20 minutes, and to nap earlier in the afternoon, or less, if you find it hard to sleep at night.

Use the evening hour for winding down

The NHLBI suggests using the hour before bed for quiet time, and avoiding intense exercise and bright artificial light, such as a TV or computer screen, during that hour. After a run of late nights, this hour is what makes an earlier bedtime realistic.

How can I protect my sleep during busy weeks?

Busy periods are exactly when sleep gets squeezed first. These habits come from the CDC and the NHLBI.

For sleep habits in general, see Want to sleep better? Which habits can you change?

When a late night really cannot be avoided

Treat it as one exception, not a new routine. Keep your usual wake-up time the next morning, take at most a 20-minute nap in the early afternoon, and go to bed earlier that evening.

What else makes me get ill easily besides short sleep?

Short sleep is one factor. Where you spend your time and what you touch matter too.

Crowds and enclosed spaces

Rush-hour trains and buses, shared offices, classrooms and shopping centres put many people close together. The CDC lists regular handwashing, cleaning commonly touched surfaces, and taking steps for cleaner air where people live and work as basic ways to reduce the spread of germs.

Hands and shared items

The UK's NHS notes that germs from coughs and sneezes can live on hands and surfaces for 24 hours. The NHS advises:

• washing your hands often with warm water and soap
• not touching your eyes or nose
• not sharing towels or household items such as cups
• catching coughs and sneezes in a tissue and putting it in the bin quickly

If you are already unwell

The CDC advises staying home and away from others when you are ill. That also gives you the chance to sleep, rather than adding another short night.

When should I see a doctor?

The NHS says most people feel better within 1 to 2 weeks. It advises seeing a doctor if:

• you have a high temperature for more than 3 days, or a very high temperature, or you feel hot and shivery
• your symptoms get worse, or do not get better after 10 days
• you have a cough for more than 3 weeks
• you feel short of breath or have chest pain
• you have an ongoing health condition that needs regular care

Also talk to a doctor if you regularly sleep more than 8 hours a night and still do not feel rested, as the NHLBI advises, or if you find yourself getting ill again and again.