Eveline von Arx: Throughout the Western world, the past few decades have seen a steady decline in the number of hours people spend sleeping.
Reto Huber: Reliable data, also from Switzerland, document that trend. Representative studies have shown that the amount of time people spend sleeping has dropped by approximately 40 minutes over the past 30 years. This holds true for both adolescents and adults. Studies also show a significant difference between weekdays and weekends – people tend to get too little sleep during the week and then make up for that deficit on Saturdays and Sundays. This is particularly common among teenagers.
RH: The sleep-wake cycle tends to shift during adolescence; teenagers go to bed late and are then tired when they have to get up early the next morning. The percentage of “night owls” increases during the teenage years and decreases only in adulthood. In adolescence, the systems that promote wakefulness are more active in the evening. U.S. studies have shown that this delayed sleep-wake cycle can have a negative effect on school achievement. Starting school later has been found to be correlated with better learning.
EvA: Are Swiss data available?
RH: We began planning relevant studies some time ago. It is important to note that a large number of factors play a role in this issue. Spending a long time on a smartphone or tablet in the evening isn’t helpful for teenagers’ sleep, particularly because of the blue light given off by LED screens. The eyes’ sensory cells that communicate information about the time of day to our “internal clock” – telling us whether it is light or dark – are particularly sensitive to this cold light, which promotes wakefulness.
EvA: Can you tell us about other factors?
RH: Parental behavior definitely plays a role. Today we know how important it is to educate parents. It matters whether parents just let their teenagers stay up late or help them get to bed earlier. Caffeine consumption can also have an effect. Teenagers who combat fatigue during the day by drinking caffeinated beverages may have trouble falling asleep at night. It’s a vicious circle.
EvA: Are there other links between sleep and performance?
RH: Studies of adults have shown that when people sleep one to two fewer hours than they actually need, performance suffers during the day. A sleep deficit of four hours increases this negative impact exponentially. These studies use what are known as vigilance tests.
EvA: Do we know why people need to sleep?
RH: There is some disagreement in the research community. We don’t really know why sleep has a regenerating effect. There are several theories: One is the synaptic homeostasis hypothesis, which is based on the idea that people’s systems are “cleaned up” while they are sleeping. During the day, connections in the brain are strengthened as new information is learned. A kind of countermovement takes place during sleep; synapses decline in both number and strength. Scientists talk about a “balancing” of synapses. A lack of adequate sleep, particularly a chronic deficit, interferes with this process.
“Our brain creates narratives, even during the night.”
EvA: What role does sleep play in learning?
RH: There is also the so-called system consolidation hypothesis. According to this view, consolidation takes place during sleep, reinforcing what has been learned. Short-term memory, in the hippocampus, receives a wealth of information during the day. During the deep-sleep phase, this information is transmitted to the individual’s long-term memory, in the cortex, and integrated into an existing network. In the morning, it is then easier to access what has been learned.
EvA: Does this process happen in the same way for everyone?
RH: In terms of qualitative aspects, not necessarily. There may well be genetic differences in how efficiently this consolidation process is carried out – just as some people are born needing little sleep and others need more.
EvA: Can you tell us more about how sleep, attention and learning are related?
RH: As I mentioned before, we have reliable data showing that people are less attentive when they are tired. A study compared two groups of sleep-deprived American students. One group was asked to operate a driving simulator, the other to listen to audio books; in other words, one group was exposed to primarily visual, the other to auditory stimuli. The researchers found that sleep was deeper in the brain region that was exposed to the relevant stimuli than in other regions of the brain. This reflects the significance of using a brain region – the more demands placed on it during the day, the deeper the sleep in that region at night. Other studies have produced similar findings. When test subjects were given problems to solve that required visuomotor skills, nocturnal sleep was subsequently deepest in the relevant brain regions.
EvA: Are there differences between children and adults in this regard?
RH: In adults, as a rule, sleep is deepest in the frontal cortex; in small children, it is deepest in the occipital regions of the brain – which is where visual stimuli are received. This is related to the fact that development in the brain proceeds from back to front. The visual systems develop first, followed by the central systems (processing of motor and sensory stimuli) and finally by the frontal cortex.
EvA: Do scientists know how important deep sleep is for regeneration?
RH: Yes. Some studies disturbed subjects during the deep sleep phase but did not wake them up – causing what is known as selective deep sleep deprivation – and found that they were less rested the next morning. We therefore believe that deep sleep is very important for regeneration. During deep sleep, at regular intervals neuronal activity in the brain is reduced to zero for a few hundred milliseconds. In a waking state, neurons are constantly active.
EvA: What topics are sleep researchers currently working on? What will they be studying in the future?
RH: Chronic sleep deprivation, for example. What happens when adolescents and adults get one to three fewer hours of sleep than they need over an extended period of time? What impact does that have? We have already conducted one study of adults that explored that question. It showed that chronic sleep deprivation changes some kinds of behavior, such as risk taking. The subjects became more willing to take risks. This finding could also have important implications for children and teenagers. Knowing more about the functions of sleep would help us understand such effects as they relate to child and youth development, and allow us to make adjustments and take measures as appropriate. It is clear, at any rate, that sleep has a positive effect on a variety of things.
EvA: One last question: Why do we dream?
RH: One explanation is that the brain is always in the process of interpreting the information it receives from the external world or from within the brain. So it creates narratives, even during the night.
Dr. Reto Huber heads a research group in the Child Development Center at University Children’s Hospital in Zurich, Switzerland. His research interests include the significance of the interaction of sleep and wake processes for behavior, learning and memory in children and adolescents.