Thomas Penzel studies sleep at the Charite Hospital in Berlin. He knows best about senile insomnia and tired ballerinas. However, that only covers two of 88 sleep disorders. We can't survive long with total sleep withdrawal, but we usually catch up on missed sleep sooner or later – with microsleep.

When a night of peaceful sleep evades you, the day ahead may be a long and dreary one

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DIE WELT: Last night I slept for eight hours and I'm still tired. What's going on?

Thomas Penzel: Maybe you aren't getting enough sleep.

Eight hours - not enough?

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Penzel: There are people who need 12 hours of sleep and others who only need four. Everyone has an individual sleep rhythm.

Then how does one test how much sleep one really needs?

Penzel: Check it out when you're on holiday. Actually, very few people know how much sleep they need. One has to be able to differentiate between fatigue and sleepiness. Fatigue means being weak and exhausted, which can also be a result of too much work. If I'm sleepy, I can hardly resist the urge to fall asleep. Sleepiness comes about from disturbed sleep or too little sleep, fatigue comes about from stress or other factors.

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Do our sleeping requirements change during the course of our lives?

Penzel: Not really. Of course, babies need sixteen hours, and during puberty adolescents also need more sleep. But otherwise it remains relatively stable.

And what about senile insomnia?

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Penzel: It's uncommon, and definitely linked to the length of sleep. Older people just spread their sleep differently during the course of the day. They have a little nap in the afternoon, for instance. From the age of twenty, the time we spend sleeping only reduces by a few minutes.

And does it makes sense to have an afternoon nap?

Penzel: Definitely. Especially when we don't get enough sleep at night. Unfortunately, however, we usually don't manage that. We recently carried out a study with the Berlin State Ballet. The dancers have the problem that they don't get enough sleep at night and as a consequence exhaustion can lead to accidents at work. That's why we looked at their sleep

behaviour. Generally the dancers are under great pressure physically and mentally to perform at their maximum level. They work to full capacity for shows and often they only get to bed at one or two. And in the morning the training starts all over again. The dancers definitely don't get enough sleep, they're sleepy and unconcentrated. They themselves, however, don't think they have a sleep deficit. Hence the accidents.

What did you suggest to them?

Penzel: We suggested they introduce a relaxation room, which they did. Behind the rehearsal room, with four beds. They are now allowed to take some time out. To lie down, listen to music, read - or sleep. The room is also used. They relax. And there are less accidents.

How long can one survive without sleep?

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Penzel: Not very long. We catch up on lost sleep sooner or later with microsleep. We carried out experiments with sleep withdrawl to see what kind of an effect it has. Reactions, memory capacity, everything slows down really fast. Already after the first night.

One can't ever not sleep again.

Penzel: Sleep is a semi-automatic system that we can paritally influence, a combination of behaviour and necessity. Just as breathing is semi-automatic. The heart is entirely automatic, whether we want it or not, the heart always beats. Breathing I can control to a certain degree. I can hold my breath, for instance. But I can't say that I'm going to stop breathing and that's it. With sleep it is exactly the same. Sleep is a necessity, but we can modulate it. We can say, tonight I want to party the night through. That works.

Your novel "Schlafes Bruder" is about committing suicide by never going to sleep again. Can one do that?

Penzel: No. Even after a long period of sleep withdrawl one wouldn't die from a lack of sleep. After a certain point the immune system would just be so run down that one would pick up infections. And that's what one would die from.

Can one also sleep too much?

Penzel: Yes, but that's very unusual. Too much sleep can lead to depression, and in turn depression can make you sleepy. It can become a difficult vicious circle to break.

Can one catch up on sleep?

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Penzel: Catch up, yes, but one can't "sleep ahead".

What are the typical complaints of people go to the sleep laboratory at the Charite?

Penzel: Today we know of 88 types of sleep disorders. They are divided into insomnia, in other words sleeplessness, and hypersomnia, in other words, sleepiness during the day. Then there are also parasomias, sleep disorders such as grinding ones teeth, nightmares, or circadian sleep-wake disorders that occur, for instance, with shift work.

And what is the most common?

Penzel: Insomnia and hypersomnia occur just as frequently, up to 20 percent of people in Germany complain about occassional sleeplessness, or rather, excessive sleepiness. However, only one to two percent of these people can be treated.

With how many patients is the cause of the disturbance mental?

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Penzel: About 50 percent of patients with sleep disorders.

Is it because for a long time sleep research was not considered a discipline in itself?

Penzel: Sleep research has been around for quite some time. In Jena at the turn of the century Hans Berger already carried out sleep research when he established that the brainwaves change during sleep. Sleep research gained particular

significance around the time of the world wars, with concern to performance and with how little sleep the soldiers could function. But that it all became sleep medicine and one said that someone who was suffering from sleepiness needed treatment, that's really new. That's only been around since1960. 1970 and only really seriously since the 90s. No one ever thought to look at sleep. People thought: Nothing happens there.

In the meantime people generally observe their sleep really intensively.

Penzel: Yes, but that isn't just because of the psyche. Civilization diseases like high blood pressure appear along with sleep disorders. Sleeping pills are also more common in the industrial countries. The observation of sleep is growing because the day is so important.

I have to function well during the day, so I also have to function well at night.

Penzel: We call that sleep hygiene. Avoid heavy meals in the evening, no or just a little alcohol, regular bedtimes, adopt rituals, write down problems, so that one doesn't carry them all to bed. Don't watch TV in bed, separate work space and bedroom. It must be clear: Now it's time to sleep.

And when that doesn't work?

Penzel: Get up, go out, do something else. And wait, until you become sleepy.

Count sheep?

Penzel: Depends. Once can. But some people miscount, get worked up and become angry. In that case its counterproductive. Then rather don't count sheep.

Is there good and bad sleep?

Penzel: Sure. During sleep the body runs through a programme.You lie awake for five to ten minutes, then you fall asleep, then the body enters a phase of light sleep, which also lasts about five to ten minutes, and then deep sleep begins, which lasts about half an hour. Then follows the so-called REM, the first dream phase. Then the whole process repeats itself. A sleep cyles lasts about 90 minutes. In between one always wakes up, but that isn't a problem. You only remember when you stay awake for longer. That's what gives you the subjective feeling of having slept poorly.

Is it really only subjectively imagined?

Penzel: We have limits. In the sleep laboratory we see how long someone was awake during the night. When it's more than 15 percent, the person slept badly.

Do sleepwalkers take off as soon as they start dreaming?

Penzel: There are two types of sleepwalkers. There is the sleepwalker who, during the dream phase, gets up and starts moving furniture, and -dangerously - can start hitting someone or inflict self-harm. That's the so-called REM sleep behaviour disorder, a newly described, very fascinating matter. And then there is the other type of sleepwalking that we recognise from storybooks - the person who always heads straight for the roof. It is a really stereotypically, monotone behaviour, like a steam engine. This occurs during deep sleep.

Should one lock the window? Is one allowed to wake up a sleepwalker?

Penzel: Of course you're allowed to, you can wake up anyone. They will just be confused. It would be better to just guide them back to bed. And locking up: Yes. There are people who can empry out a fridge during the dream sleep phase.

Has a sleepwalker ever killed someone?

Penzel: There are at least trials about that. Someone claimed to have shot someone while sleepwalking. There are then studies, to see whether the person is really a sleepwalker or if that person is just making it up. When that's the case and the person really tends toward complex motion sequences, its an exculpatory argument. Despite that, of course no one is allowed to kill anyone. The person will probably be confined to a psychiatric clinic.

Are there people who can sleep while they are standing?

Penzel: Yes, falling asleep on your feet can happen. But you don't enter a deep sleep. At a certain point your muscles relax so much that you just collapse.

Sleep and death are often compared - rightly so?

Penzel: I personally find that really bad. As a consequence of this sleep was neglected by science for a long time. The doctor examined sick patients during the day but never at night. But it can't work like this.

Is dying like falling asleep?

Penzel: Only if you're really lucky.

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