Chapter 8:1 Introduction

Sleep is such an important part of our lives that a lack of it strongly correlates with negative outcomes on nearly every measure of health. People who sleep less than approximately 7 hours a night are at a greater risk for heart disease, stroke, asthma, arthritis, depression, and diabetes. Nearly 20% of all car crashes, both fatal and nonfatal, are attributed to drowsy driving. The cancer research branch of the World Health Organization has determined that disruption of regular sleep is “probably carcinogenic to humans,” putting it in the same risk category as the infectious agents malaria and human papillomavirus (HPV), as well as the biochemical weapon mustard gas. Sexual health is affected by sleep deprivation as well, as men with the worst sleeping habits have significantly lower sperm counts, decreased circulating testosterone, and even testicular shrinkage.

Despite all that we know about the benefits of sleep, sleep is often the first time commitment to get cut, often getting squeezed as people stay awake later while waking up sooner. Consider that the CDC estimates that more than a third of American adults fail to get enough sleep each night. Almost 70% of college students fail to get the recommended amount of nightly sleep, and half of all college students report experiencing daytime sleepiness as a result.

The current medical recommendation is 7-9 hours of sleep each night. But why is sleep so important? It is possible to study sleep using a combination of techniques; the output of sleep studies are visualized on a polysomnogram (pahl- e-SOM-nuh-gram; somn- is the prefix referring to sleep). Several physiological measures are taken in a polysomnogram, including heart rate, blood pressure and oxygenation level, respiratory depth and pattern, muscle activity, eye movement, and one of major interest to neuroscientists, brain wave activity. While everyone knows what sleep is, it is useful to try to more precisely define sleep as a biological function. Sleep is characterized by the following:

A decrease in physical activity

This is not to say that people do not completely cease all movement during sleep. It is very common to readjust posture many times in the middle of the night. Some people may grind their teeth together or talk in their sleep, sometimes carrying on full conversations by themselves! About 15% of people have experienced somnambulism, or sleepwalking: full on wake-like behaviors such as navigating down a flight of stairs or preparing a sandwich, performed entirely in the absence of conscious awareness. Despite these rare occurrences of physical activity, the average movement of the person over a night’s rest is still less than their average activity when awake.

Figure 8.1 Sleep is usually characterized by a decrease in physical activity, but some people experience somnambulism. CREDIT: Delacroix, Eugene. Lady Macbeth Sleepwalking. 1849-1850.

A decoupling from external inputs

When we sleep, our conscious brains are “distanced” from the outside world. Sleep causes a heightened threshold for detection of stimuli, so we do not receive the same magnitude of inputs from our sensory systems as when we’re awake. This is why someone else might have to talk loudly or even shake you physically to wake you up.

Changes in brain wave activity

Sleep was once thought to be a period of time characterized by low brain activity. After all, the person looks like they are not moving. Shouldn’t brain activity be reflective of that decreased state of activity? With the advancement of EEG technology in 1924, and the rise of sleep laboratories in the 1970s, scientists who studied brain activity noticed that, at different times throughout the night, the brain of a sleeping person was very similar in activity to the brain of an awake person!