The morning after a short night
The alarm goes off, and it feels as if you have only just lain down. Last night got late, first the series, then your phone, and at some point you were still lying awake going through the next day in your head. Now your eyelids are heavy and the first coffee barely works. Over the course of the morning you read the same message three times, a name you actually know escapes you, and some small thing winds you up more than usual. In the afternoon you reach for the chocolate without really noticing why.
The body copes well with a night like this. After a restful night, the whole thing is usually over. It becomes a problem when the short night turns into a habit. That is when it becomes clear that sleep is much more than a break. Your chronological age counts the years since you were born. Your biological age, on the other hand, describes the actual condition of your body, for example how well its cells renew themselves and how fit your brain stays. And that is exactly what your body works flat out on at night. So what happens while you sleep?
The night shift cleaners: What your brain takes care of while you sleep
Picture a large kitchen in a busy restaurant. Cooking goes on all evening, pots pile up and scraps end up in the corners, because there is no time for thorough cleaning. Only after the kitchen closes do the cleaners come in, push the tables aside and wash everything down. If they skip a night, the kitchen is a little less clean the next day than the day before.
Your brain is in a similar situation. Its nerve cells work all day and produce waste products in the process. These include beta-amyloid, a protein that clumps together in the brain in Alzheimer's disease. Unlike the rest of the body, the brain has no lymphatic system of its own to carry away such residues. Instead, it uses the fluid that surrounds it.
A research team led by Danish neuroscientist Maiken Nedergaard discovered how this works in mice in 2013. During sleep, the spaces between the brain cells widened by around 60 percent. This allowed the brain fluid to flow through much more easily, and beta-amyloid was cleared away faster than during waking hours. A few years later, researchers in Boston saw something similar in humans. They had 13 young adults sleep in an MRI scanner and observed how, during deep sleep, a wave of brain fluid passed through the brain roughly every 20 seconds.
So the cleaning needs deep sleep. But what happens when it is skipped?
A wild night for the brain
A team at the US National Institutes of Health (NIH) set out to answer this question. 20 healthy people aged between 22 and 72 came to the lab twice: once after a good night's sleep, and once after staying awake for around 31 hours. Both times, the researchers used a special scan to measure how much beta-amyloid was in the brain.
After the sleepless night, the level was about five percent higher, and of all places in the hippocampus, which is central to memory, and in the thalamus. Of course, a single night does not make anyone ill, and the body catches up on some of this over the following nights. But the experiment shows how directly sleep and the cleaning of the brain are linked.
A long-term study from London shows what this can mean over decades. Almost 8,000 British civil servants reported how long they slept over many years. Those who regularly slept around seven hours at 50 and 60 were less likely to develop dementia later than those who got six hours or less for years. The difference was around 30 percent.
Sleep is not wasted time. It is the time in which your brain is looking after the decades to come.
More than the brain: What sleep means for your whole body
The night-time cleaning is not the only thing that happens during sleep. During deep sleep, the body releases a large part of its growth hormone, which is involved in repairing tissue and muscles. The immune system uses the night too. In an experiment in San Francisco, researchers dripped cold viruses into the noses of 164 volunteers. Those who had slept less than six hours a night in the week before became ill considerably more often than those who had slept more than seven hours.
If sleep is lacking over a long period, the level of silent inflammation in the body also rises. It keeps the tissue in constant repair and makes the telomeres, the protective caps at the ends of the chromosomes, shrink faster. Read more here (Article: Biological age). So good sleep acts on the same processes that determine your biological age.
How much sleep do you need?
For adults, the best range is seven to nine hours. A large analysis of 79 studies with over two million people shows: those who sleep within this range have the highest life expectancy on average. Both considerably less and considerably more sleep go hand in hand with a shorter one.
With long sleep, it is worth taking a closer look. People who regularly need more than nine hours and still do not feel rested often have another reason for it, such as an illness that is draining their strength. In that case, long sleep is more of a warning sign than the cause. That is a good reason to have it checked by a doctor.
How much sleep you personally need is partly innate. A good rule of thumb: if on days off you wake up without an alarm at roughly your usual time and do not constantly have to fight tiredness during the day, you are probably getting enough sleep.
Regular times: The underrated part of good sleep
Besides duration, there is one factor hardly anyone thinks about: how regularly you sleep. Your body has an internal clock that controls hormones, body temperature and digestion over the course of the day. It works best when you go to bed and get up at roughly the same times. If you go to bed at eleven during the week and at two at the weekend, for your internal clock it is as if you flew to a different time zone every Friday.
A study from the UK shows how much difference this makes. More than 60,000 people wore a motion sensor on their wrist for a week that recorded their sleep. Over the following years, those with a regular sleep rhythm lived longer on average than those with the most irregular one. Remarkably, regularity said more about life expectancy than sleep duration did.
For your biological age, what counts is not only how long you sleep, but also how reliably.
What promotes good sleep
The most effective lever is a fixed wake-up time, including at the weekend. It sets your internal clock, and falling asleep in the evening often follows by itself. Daylight in the morning helps too, for example a short walk outside, because bright light is the strongest signal for your internal clock.
Coffee, cola and energy drinks keep working for many hours. If you have trouble falling asleep in the evening, it is best to avoid them from early afternoon onwards. Alcohol may help you fall asleep faster, but it makes the second half of the night more restless and your sleep less restorative. The bedroom should be cool, dark and quiet, and the last hour before going to bed can be calmer than the rest of the day. Charging your phone outside the bedroom helps many people with this.
And if you lie awake at night brooding, it is better to get up after about 20 minutes, read something in dim light and only go back to bed when you feel tired again. That way, your brain learns to associate bed with sleep and not with brooding.
You don't have to change everything at once. Start with one thing, ideally the fixed wake-up time, and give your body a few weeks to get used to it.
Where you stand on sleep
How long and how well you sleep are two of the questions in the BioAge test. Together with the other questions, they produce an estimate of your biological age and an overview of where you have the most to gain.
If you sleep badly for weeks, snore loudly and stop breathing for short periods, or can hardly stay awake during the day, have it checked by a doctor. For persistent problems falling or staying asleep, the current German guideline considers a special cognitive behavioural therapy for insomnia the first-choice treatment, even ahead of sleeping pills. In Germany, it is now also available as an app on prescription (a so-called DiGA).
Frequently asked questions about sleep and biological age
Can I catch up on sleep at the weekend?
Partly. In a large Swedish study, people who slept little during the week and longer at the weekend did not live any shorter on average than those who got enough sleep. Large shifts do, however, throw your internal clock out of sync. Several slightly longer nights are better than a single very long morning.
Is a nap good or bad?
A short nap of 10 to 20 minutes can noticeably improve concentration in the afternoon. If you sleep longer, you slip into deep sleep and often wake up groggy. If you have trouble falling asleep at night, it is better to skip the nap.
Do sleeping pills or melatonin help?
Sleeping pills can help in the short term during acute phases, but they are not suitable for the long term because the body gets used to them. For ordinary sleep problems, melatonin has only small effects and helps more with jet lag. For persistent problems, behavioural therapy is the better choice.
Does exercise help you sleep?
Yes. People who exercise regularly sleep better and more deeply on average. However, some people are still wired after a very intense workout right before bed, in which case it is better to train a little earlier. Read more here (Article: Endurance).