Initially when we close our eyes we lightly drift in and out of sleep and can be awakened easily by external noise. Muscle activity slows down. The eyes move slowly and occasionally we see flashes of images which may be visual thoughts. There is also the possibility that we may be awakened suddenly by a sensation that we are falling or something similar. This is called stage 1 sleep.

In the second stage of sleep, eye movement stops altogether and brain waves slow down. Slower brain waves called delta waves begin to appear in the third stage of sleep and it becomes more difficult to wake someone up. This is the transition into deep sleep. During stage 4 (slow wave or delta sleep) the brain mainly produces delta waves. This is more intense than in Stage 3. There is no eye movement or muscle activity. The body begins to repair itself.

The final stage is REM sleep. The eyes move around in different directions and muscles are paralysed. We breathe more quickly and our heart rate increases. This is the stage where most of our dreaming occurs. The first REM period usually occurs around 90 minutes after we fall asleep and is normally short. We would normally have around three to five REM periods in a night. The length of each REM period increases as the night progresses while the periods of deep sleep (stage 4) decrease. The longest REM period is therefore in early morning or the last one we have before we wake up. On average this usually lasts around 30 minutes. This is when we are most likely to remember our dreams.