The Science of Sleep

Whether you are awake or asleep is determined by two groups of nerves in the brain. Each group acts like an electrical circuit. One circuit of nerves controls being awake, and another circuit controls sleep. We should think of the two circuits as acting like a light switch. When we are awake, the “wake” nerves are active and are firing signals to shut down the sleep circuit. When we are asleep, the “sleep” nerves are active and shut down the “wake” circuit.

At the start of the day, our ‘wake nerves’ control the switch and we are able to get up and get to school or work. As we stay awake during the day, special “tiredness chemicals” build up in the brain which flick the switch and turn control over to the sleep nerves. Some of us drink coffee during the day because caffeine can block the tiredness chemicals from working. This means we can delay flicking the switch to activate our sleep system.

The sleep nerves create different types of brain activity that control different functions and scientists call these “stages”. Sleep stages can be light, deep, or have ‘rapid eye movement’ dream sleep (REM).

Sleep Stages

Light sleep.

Once you close your eyes, your brain activity slows down. Stage 1 sleep occurs for about the first 5 minutes and it’s easy to be disrupted by light or noise during this time. During stage 1 sleep, you lose your awareness of your surroundings, lower your body temperature, and relax your muscles. If you are woken up during this type of sleep, you might think that you were only “drifting off” and not fully asleep. Stage 2 comes next and lasts for approximately 25 minutes. During this time, your brain activity becomes irregular, your heart rate and breathing rate decrease, and if awakened, you would know that you were asleep. About half the night is spent in stage 2 sleep.

Deep sleep.

As sleep becomes deeper, less and less of the brain remains active and brain activity starts to become rhythmical. Sometimes this is called “slow wave sleep”. During deep sleep, your breathing becomes deep, slow and regular. Your blood pressure falls up to 25 percent from the waking pressure. It’s much more difficult to be woken out of deep sleep. Growth hormone is released at the beginning of deep sleep to repair your body, and the immune system is strengthened. About one fifth of the night is spent in deep sleep.

Dream sleep.

Dreaming only occurs during Rapid Eye Movement sleep (REM sleep). During REM, your brain activity is similar to being awake, but the brain centers which control body movement have been turned off. During REM sleep, your body temperature rises, your blood pressure and heart rate increase, your breathing speeds up and your sexual organs become erect. The body system which controls your ability to take “flight or fight” is twice as active as when you’re awake, so it’s a good thing the brain provides temporary paralysis to the muscular system. REM usually occurs 4 or 5 times a night and provides the brain the ability to do things like interpret emotional information from the day, or to transfer learned skills to memory. This explains why students who stay up all night cramming for a test, do not retain information as well as those who get a night of sleep.

References

Saper CB, Chou TC, Scammell TE. The sleep switch: hypothalamic control of sleep and wakefulness. Trends Neurosci. 2001;24:726-731.