We speak to Dr. Christine Blume, sleep researcher at the Centre for Chronobiology at the University of Basel, Switzerland. Her research centres around the factors that influence sleep in modern societies and she is currently investigating the effects of evening screen light on sleep.

WHAT IS SLEEP?

Sleep is one of the most consistent behaviours we observe in humans: Around the globe, irrespective of culture or age, humans spend about one third of the day in this state of diminished responsiveness. This behavioural change is paralleled by major changes in brain activity and associated physiological processes. During sleep, the chit-chat of neurons becomes slower and more harmonic, like a choir suddenly starting to sing in tune. But just like the verses of a song, the activity of the sleeping brain is not always the same. Rather, four so-called ‘sleep stages’ are repeated about four to five times per night. Each of these repetitions is called a ‘sleep cycle’. When we fall asleep in the evening, we first enter light sleep. This state comprises two stages, so-called N1 and N2 sleep, where the ‘N’ denotes that these are ‘non-rapid eye movement’ sleep stages. More specifically, N1 is a transitory state between wakefulness and sleep and only accounts for about 5% of a night’s sleep. When woken up from N1 sleep, people are sometimes not even sure whether they were asleep. In contrast, N2 is often considered the first ‘real’ sleep stage. It is also the stage in which we spend about 50% of the night. Following N2, we enter deep or N3 sleep, a state characterised by very slow brain activity and maximal relaxation. The majority of deep sleep usually takes place in the first half of the night with N3 episodes becoming shorter or even missing in the second half. On average, N3 amounts to about 20% of the night, although this percentage decreases with age. A sleep cycle is concluded by a phase of rapid eye movement or REM sleep. As its name says, this stage is characterised by rapid horizontal eye movements besides almost wake-like brain activity and a very low muscle tone. Although we nowadays know that we dream during all sleep stages, the most vivid and bizarre dreams take place during REM sleep. This sleep stage amounts to 25% of the night and just like deep sleep, the proportion decreases the older we get. Most of the REM sleep occurs in the morning hours.

WHY DO WE SLEEP?

The famous sleep researcher Allan Rechtschaffen once said “If sleep does not serve an absolutely vital function, then it is the biggest mistake the evolutionary process has ever made.“ Indeed, while sleep looks and often feels like a unitary state of ‘inactivity’, many essential processes go on behind closed eyes. First, sleep helps to restore a state of ‘normality’ and replenish resources that are used up during the day. Everyone probably knows from experience that sleep is essential for wellbeing and performance – or more generally normal functioning of our body and mind. Indeed, during sleep metabolic waste products that accumulate during daytime are cleared from the brain and it greatly benefits the consolidation of fresh memory traces. Moreover, sleep benefits a healthy energy balance as well as tissue regeneration and repair. Unsurprisingly, habitual short sleep, that is, less than six to seven hours per night, is therefore associated with adverse health outcomes. For example, chronic sleep disturbances in shift workers are associated with increased cancer rates and the risk of developing metabolic disorders such as type II diabetes. Furthermore, habitual short sleepers are at a higher risk for infectious diseases, which underlines the importance of sleep for the immune system.

HOW MUCH SLEEP DO I NEED?

The rule of thumb is that healthy adults should sleep between seven and nine hours per night. However, the amount of sleep someone needs is highly individual. And in fact, rather than asking someone, we should ask out body as it knows best how much sleep it needs. Ideally, we go to bed when we are tired and wake up without an alarm clock. Knowing that, at least during the week, this does not work for most of us, this is the only way to find out your sweet spot – perhaps during the next holiday? Generally, sleep experts agree that many people in modern societies do not sleep enough, at least on workdays. This is underlined by research during the Corona pandemic, which has shown that the increased flexibility of working hours and the place we work from have freed time for sleep. Thus, people globally slept more during lockdowns and stay-at-home orders.

SHOULD WE PRIORITIZE SLEEP MORE?

Many people probably still underestimate the importance of sleep for performance and health. Thus, we should overcome sleep ‘machismo’ and instead promote behaviour that is likely to benefit healthy sleep. We need to understand that wakefulness requires sleep and vice versa – they are two sides of the same coin and should thus be equally important.