“Most of the teenagers would have a benefit, when school would start later”

Sleep is a physiological human need, comparable with the need for food, water and air. But how much sleep do we need? And what happens, if we don’t get enough? FQF talked to Alfred Wiater, chairman of the board of the German Sleep Society (DGSM), and Thomas Penzel, Professor at the Charité university hospital in Berlin, Germany, about sleep.

Dr Wiater, Professor Penzel, it is uncontroversial that human beings have to sleep to survive. Could you please point out the key functions of sleep?

Key functions are energy conservation, physical and mental function restauration. This includes hormone secretion, immune functions, memory and mood functions. Regarding energy conservation, during sleep energy is reduced to maintain body functions at a resting state. Physical restauration means that cell repair mechanisms dominate their acitivity. Mental function is much related to memory and learning. We know that during sleep it is decided what will remain in long-term memory and what will be forgotten. Immune functions are important: when we have fever we need to sleep more. And vice versa, if we sleep to short or too little, we get infections much more easy.

Depending on people’s age, the need for sleep notably alternates. Is it more or less a general rule, that adults require between 7 and 9 hours sleep per day, infants need much more and elderly people get by with less?

This is partially correct. While the newborn baby needs up to 16 hours sleep, this decreases until adolescent. After the age of 20 sleep does not decrease much any longer and we need between 7 and 9 hours of sleep. Beside that, sleep duration is quite stable for an individual. Some people need less sleep than others. We call them short sleepers, if they need less than 6 hours and long sleepers, if they need more than 9 hours. However, these are just a few percent of the population.

Professor Thomas Penzel

Which health effects arise from a long-term lack of sleep?

Lack of sleep makes us tired. Reaction times get longer. Reactions on tasks take longer and get more flawed. Performance in general, mood and memory get worse and we are more susceptible to infections, metabolic and cardiovascular diseases. If reaction times become longer, this may be quite difficult, because if this happens to a sleepy driver, this is dangerous and may cause accidents. If reactions are not adequate to the situation, than a flawed reaction will cause accidents as well. I mentioned already the tendency to have more infections when sleeping too short. And of course, memory is impaired because sleep is important for memory sorting functions.

In many countries, school starts around 8.00 o’clock in the morning or earlier. It is said, that teenagers, particularly in the age of puberty, prefer to go to bed late. When they have to get up too early to go to school, the lack of sleep reduces their performance capability. That’s why several scientists claim that school start times should shift to 8:30 a.m. or later to make it compatible with the teenagers’ sleep habits. What do you think about it?

There is a shifting of circadian rhythm during puberty, which is partially behavior and partially hormone driven. Therefore, most of the teenagers get tired later. When they go to bed too late, then they may not sleep enough and getting up for school might be too early for having enough sleep. Yes, this is right. Therefore, most of the teenagers would have a benefit, when school would start later.

Lastly, many people get tired about noon, particularly after lunch. What’s your opinion regarding power napping?

Power napping is a good idea, if work schedule allows this to do. It will help to be more alert in the afternoon. To be more tired after lunch is quite natural, it is part of our circadian rhythm, internal clock regulation. This behavior is still persisting in many mediterranian countries and is found in retired people, who are not bound to our western style work-lunch-break rhythm, which does not allow power napping.

Interview: Sebastian Kaiser


Dr. Alfred Wiater has studied human medicine at the University of Bonn, Germany, where he received his approbation as a physician in 1977 and completed his doctorate in 1978. He continued education in pediatrics at the Children’s Hospital of the St. Bernward Hospital in Hildesheim and worked as assistant doctor in the Hildesheim Regional Hospital. He received his recognition as a doctor for pediatrics in 1982 and continued his career as a senior physician in the Department of Pediatrics at the Johanniter-Kinderklinik in St. Augustin. In 1990, Alfred Wiater became chief physician of the Children’s Hospital of the Porz Hospital in Cologne and lecturer at the University of Cologne. Since 2012, he is chairman of the board of the German Sleep Society (DGSM).


Prof. Dr. Thomas Penzel graduated from physics (1986), human biology (1991), and physiology (1995) at the University Marburg, Germany. He received a certificate for sleep medicine (1997 – DGSM, 2013 – ESRS), for medical informatics (1997) and in 2001 became Professor at the University of Marburg. In 2006 he moved to Berlin as scientific director of the interdisciplinary sleep medicine center at the Charité university hospital. He received several awards (Bill Gruen Award for innovations in Sleep research, Bial Award for Neurotelemedicine, Distinguished Award for Development of Sleep Medicine and Sleep Research in China) and is member of the board of several societies (WASM, WSS, DGSM, DGBMT) and journal boards. He published more than 250 journal papers, edited and coedited more than 10 books. His research is on sleep research, sleep medicine, cardiovascular system, sleep disordered breathing and biosignal processing.

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