Evolution of Sleep and Memory
Kazuhiko Kume, Kumamoto University
Sleep is one of the most familiar behaviors to us. Busy scientists like us tend to wish we could skip sleep so that we could work 24 hours a day. But the fact is all of us do sleep several hours every day, and we wonder 'why' we need sleep. However, much is still unknown, and it is only recently that scientists started to understand 'how' and 'why' we sleep with rapid arise in novel animal models and molecular biology. Thus this is one of the frontier topic of this field.
Since the development of electroencephalogram (EEG, brain wave), human and animal sleep have been studied scientifically. Mammals and birds have two completely different types of sleep, namely NREM and REM sleep.
NREM sleep is further divided into 4 stages according to its deepness. NREM sleep, especially deep NREM sleep, is characterized by slow and high amplitude brain waves, which is distinct from those of arousal state. During NREM sleep, the metabolic rate of the brain lowers, the body and the brain temperature goes down, consciousness is completely lost, dreams are rare and monotonous, if present, sensory inputs are blocked and shut off at the entrance level (hypothalamus) and stronger stimulations, a longer time and more pains are required to wake up from deep NREM sleep.
As its name indicates, REM sleep is characterized by rapid eye movements. During REM sleep, the motor outputs are blocked at the exit level (pons), so that our body is almost motionless except for eyes, the brain waves are similar to those of awake state, the metabolic rate of the brain is also equivalent to, or even higher than arousal, dreams are frequent and often vivid, and it is easy to wake up from REM sleep. Both NREM and REM sleep are necessary to keep the best of various physiological systems, especially nervous functions, and both of them are regarded essential to the survival.
Sleep of animals varies largely between species. Some of grass-eating animals such as cows sleep only for a few hours a day, while sloppies sleep almost all day. But most animals, both diurnal and nocturnal, sleep occasionally all through the day when possible, like human babies. Adult human sleep is unique in that it is well consolidated and occurs only during night time. Another interesting point of our sleep is the fact that we can control our sleep by our intention or consciousness. Even when we feel 'sleepy' during the lecture or meeting, we can resist against our sleepiness, and sometimes we win, sometimes we are defeated and doze off and our consciousness are gone. Thinking about 'who' fight against 'whom' in our brain when we are sleepy is quite an exciting and interesting question. It may lead us to a very hard philosophical question on 'who we are' or 'self', which I regretfully avoid to pick up as a topic during this session.
Many other animals including non-vertebrate species become the subject of sleep study recently. Fruit flies are intensively studied to date. They do not have NREM and REM types of sleep, but their sleep-like resting states show similarities with mammalian sleep in that they are under both circadian and homeostatic regulations, and they are accompanied by increased arousal threshold, meaning they are less responsive to sensory inputs. In addition, they use dopamine to regulate sleep and arousal just like mammals, indicating that insect and mammalian sleep derived from evolutionarily the same origin of behavior. It should be noted that fruit fly sleep is also important for a long life span, and memory.
The conservation of a similar behavior among species may suggest its physiological role. Animals need rest after physical exercise as a restoration process. But it should not necessary be accompanied by sleep. Sleep is regarded primarily as a rest for brain. Thus depriving sleep or insufficient sleep result in the low performance of the brain function, such as vigilance and intellectual abilities on the following day. Moreover, sleep is known to have additional functions. During sleep, it is reported that our memory is rearranged, enhanced and sorted. Sleep apparently looks costly behavior since animals become unsafe and do nothing. But this type of positive function of sleep might have been crucial for evolution of high intellecual ability of higher animals especially human being.
We will discuss what is known about the mechanism of sleep particularly focusing on its relationship with memory, and consider why it developed evolutionarily.