Sleep Stages

Scientists divide human sleep into four stages: 1, 2, 3 and REM. They can detect these stages through special wires that attach to the scalp on one end and a machine known as EEG (electroencephalogram) at the other end. Sleep is compromised of four to five cycles, each lasting approximately 90 minutes and each is made up of a mixture of the different sleep stages.

The awake brain: While you are reading this, your brain is working fast, which is reflected by the fast activity on the EEG. This activity is called alpha waves.

Stage 1: When you go to bed and close your eyes, you are still awake but soon afterwards, your brain drifts into a drowsy state where you may experience some fleeting images or sensation. They are called hypnagogic hallucinations. You might have a quick jerk as you fall asleep- called sleep starts or hypnic jerk. It is benign and lots of people have them. Soon, you enter stage 1, which is “light sleep” which means you can easily be awakened; you might spend 5 to 10 minutes in stage 1 before you move on to other stages. The brain waves during stage 1 are called theta waves. If you wake someone from stage 1, they might not remember that they slept at all.

Stage 2: This is the most common stage of sleep. You probably spend 50% of your sleep in this stage (again in cycles).  Here you are entering into the deeper stages of sleep and the brain produces beautiful looking waves called spindles and K complexes. Your body’s physiology begins to slow down- such as temperature, heart rate and breathing.

Stage 3: This is where you truly enter the deep stages of sleep. The EEG shows big and slow waves called delta waves or slow waves. It is more difficult to wake someone from this stage. Sleep walking occurs out of this stage in some people.

Stage REM (rapid eye movement): You typically enter REM sleep about 90 minutes of falling asleep. Most of dreaming occurs in REM sleep. We spend about 25% of our sleep in REM sleep. Your body’s physiology does some weird stuff during this stage. The eyes move very fast, breathing becomes irregular but most interestingly, there is an increased activity in the limbic (emotional) and occipital (visual) regions of the brain, resembling the active wake stage.  Your brain is busy here. This is why REM sleep is also called paradoxical sleep. Your muscles become super relaxed (nearly paralyzed) but there is dreaming, visual imagery and heightened emotional states. People with narcolepsy can have REM sleep soon after they fall asleep or even during awake which is abnormal.  Nightmares typically occur out of REM sleep.