Many of us think of sleep as a time when the mind and body shut down, but in fact what happens when our head hits the pillow is quite the opposite. Sleep is a dynamic process – our brain changes its state many times as we pass through the five stages of sleep in approximately 90-minute cycles. The first four stages of sleep make up our non-rapid eye movement (NREM) sleep and the fifth stage is when rapid eye-movement (REM) sleep occurs.
Across NREM sleep we move from very light sleep during Stage 1 to very deep sleep in Stage 4. It’s very difficult to wake a person who is in Stage 4 sleep. Typically, our eyes do not move during NREM and we have low muscle activity, although all of our muscles are still able to function.
Stage 1 – Very Light sleep: We drift in and out of sleep and can be awakened easily.
Stage 2 – Light sleep: Where eye movement ceases.
Stage 3 & 4 – Deep sleep: We’re difficult to wake and have no eye movement or muscle activity.
Stage 5 – REM sleep
During Stage 5 or REM sleep our brain waves are as active as when we are awake and breathing becomes more rapid. The limb muscles are temporarily paralysed, our body does not move, eyes can dart rapidly in all directions and we dream vividly.
A typical night under the covers isn’t simply four to six of these 90-minute sleep cycles pieced together. In the first two to three cycles of shut-eye we spend most of our time in deep Stage 3 and Stage 4 (NREM) sleep. During the final two to three cycles we enter more REM sleep which is accompanied by some lighter NREM sleep.
Cycling through the sleep stages is important for preventing tiredness and irritability the next day and maximising the benefits of sleep. Next up, we’re taking a look at how sleep can boost performance. Stay tuned!