Science Fact of the Week 40 - Sleep

Sleep is a physical and mental resting state in which a person becomes relatively inactive and unaware of the environment. In essence, sleep is a partial detachment from the world, where most external stimuli are blocked from the senses.

Normal sleep is characterized by a general decrease in body temperature, blood pressure, breathing rate, and most other bodily functions. In contrast, the human brain never decreases in activity. Studies have shown that the brain is as active during sleep as it is when awake. Throughout an eight-hour sleep cycle, a normal adult alternates between two very different states, non-REM and REM (Rapid Eye Movement) sleep. Non-REM sleep consists of four stages that range from light dozing to deep sleep. Throughout this state of sleep, muscle activity is still functional, breathing is low, and brain activity is minimal. Approximately 75% of the sleep cycle is spent in non-REM sleep.

Most dreaming takes place during REM (Rapid Eye Movement) sleep. Periodic eyelid fluttering, muscle paralysis, and irregular breathing, body temperature, heart rate, and blood pressure distinguish REM from non-REM sleep stages. REM sleep is also called "paradoxical" sleep because brain wave activity is similar to an awakened state. It is during REM sleep that the brain blocks signals to the muscles to remain immobile so dreams will not be acted out. Adults spend about 20 - 25% of their sleep cycle in REM sleep.

So why do we sleep? Some believe that sleep gives the body a chance to recuperate from the day's activities but in reality, the amount of energy saved by sleeping for even eight hours is miniscule - about 50 kCal, the same amount of energy in a piece of toast. We have to sleep because it is essential to maintaining normal levels of cognitive skills such as speech, memory, innovative and flexible thinking. In other words, sleep plays a significant role in brain development.

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