Sleep appears to be almost universal in the animal kingdom; all of us do it in one form or another. Smaller animals like cats sleep up to 12 hours while large mammals like giraffes can be refreshed after a mere 90 minutes. As humans we fall somewhere in the middle of that spectrum. We spend about one third of our lives sleeping, and it's an activity essential as diet and exercise to overall health.

A lot happens in our bodies while we sleep — it's almost as if it's time for the brain to get to work!  

A complete sleep cycle takes 90 to110 minutes. Most people go through three-to-five cycles of sleep each night lasting a total of eight hours. Here's a look at the stages of each of those of those cycles.

Stage 1: Falling Asleep

This is the period of time when you drift in and out of sleep. Your eye movements and muscle activity begins to slow. Some people experience a sensation of falling while others have hallucinations. This phase only lasts for five to ten minutes and isn't repeated unless you wake up for a period of time in the middle of the night. 

Stage 2: Light Sleep

Eye movement stops, brain waves become slower and your body temperature drops. Generally this phase lasts about 20 minutes but over the course of the night, most people spend about 45-50 per cent of the night in light sleep.

Some people say that if they wake up during light sleep, they feel energetic and refreshed. Sleep tracking apps are one good way to learn about your sleeping patterns and ensure that the alarm goes off at the perfect time.

Stage 3: Deep Sleep

Your brain waves are extremely slow and there is no eye movment or muscle activity. It's really difficult to wake up from deep sleep and it's often when children experience bedwetting or night terrors.

Pardoxically, it's also when sleep walking occurs. Anywhere from 1 -15 per cent of us exhibit these "parasomnias" or unsusual behaviours known to happen in deep sleep.

Deep sleep periods last approximately 30 minutes, with progressively shorter periods with each sleep cycle.

Stage: REM (Rapid Eye Movement) Sleep

Interesting fact: REM sleep only appears to have evolved with mammals.

During this part of the sleep cycle, breathing becomes more rapid and shallow, heart rate increases, blood pressure rises and the body temporarily loses its ability to regulate temperature. The eyes start to jerk rapidly and the brain is so active it almost appears awake.

At the same time, limb muscles are temporarily paralyzed. Have you ever had the feeling like you're trapped in a dream? That's because you are; your most vidid and complex dreams occur in REM sleep when your body is unable to move.

We spend 10 minutes during the first cycle of REM sleep with progressively longer periods during each sleep cycle.

REM is particularily important to babies who spend 50 per cent of their time in REM, whereas adults spend only 20 per cent. 

Nightime waking sometimes happens after the REM cycle, but if the waking period is short, you won't remember the next morning. It's the reason people tend to forget middle of the night phone calls or hitting the snooze button on their alarm clock. 

For more about sleep, watch While You Were Sleeping on The Nature of Things.

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