Introducing sleep Most people sleep over 175,000 hours in their lifetimes. This can vary between people, depending on their sleep habits. It can be quite depressing to think of how many hours wed save if only we could sleep less. The fact that we spend so much time sleeping suggests that it must be good for something But what is sleep actually good for? Why Do we Sleep? Does sleep fulfill some vital physiological role? Ought to look similar across lots of animal species However, animals tend to have very different sleep patterns. Donkeys may only sleep 23 hours per day. Opossums over 18h. Humans fall near the lower end of the spectrum, sleeping an average of 69 hours per night. Nonterrestrial mammals (whales, seals, walruses etc.,) have unusual sleep patterns. In certain cases, their version of sleep happens one hemisphere at a time. Species differences in sleep patterns is our first hint that there is more going on during sleep than just rest. Stages of sleep Early researchers studying the brain during sleep made two interesting discoveries. 1. The brain is quite active during sleep, at certain times showing levels of electrical activity that are equivalent to being awake. Prior to this discovery, it had been assumed that the brain went into a state of dormancy or inactivity during sleep. 2. The brain goes through several discrete stages while sleeping. Sleep is heterogeneous with respect to the brain. Prior to this discovery, sleep was considered to be a unitary, onoff function. Using a machine called an electroencephalogram (EEG), it is possible to record the brains electrical activity using electrodes placed on the skull. When a lot of neurons are firing separately, one sees a lowamplitude, highfrequency pattern of EEG waves. When neurons are firing in a synchronous manner, the frequency of the EEG wave goes down, but the amplitude goes up. When the individual is awake and fully alert, the EEG shows beta waves. Beta waves are highfrequency, lowamplitude waves. During nonREM (also called deep sleep), the individual shows delta waves on the EEG. Delta waves are lowfrequency and highamplitude. During Rapid Eye Movement (REM) sleep, brain activity looks very similar to how it does while awake. REM sleep is characterized by highfrequency, lowamplitude waves. NonREM sleep NonREM sleep, also called deep sleep, is a time during which a large range of activities take place. For example: body temperature decreases, growth hormone secretion increases.