What sleep is

1. Sleep is a natural, reversible state in which your awareness of the world outside drops away and your body goes still.
2. Brain activity carries on right through the night, and in some stages it runs almost as high as it does when you are wide awake.
3. Sleep has three benefits: a healthy brain, physical repair and emotional stability.

The three benefits of sleep

1. A healthy brain
   1. While you sleep the brain consolidates what you took in during the day, so it settles into long-term storage instead of fading.
   2. Waste products that build up in brain tissue while you are awake are cleared out overnight.
   3. After a full night you can hold attention through a lesson and recall what you revised the evening before.
2. Physical repair
   1. Growth hormone is released during the deep stages of sleep.
   2. Growth hormone drives the repair of damaged cells and the growth of muscle and bone.
   3. Energy stores are topped back up and the immune system works better, so you pick up fewer infections.
3. Emotional stability
   1. Sleep gives the brain areas that handle emotion a chance to settle, so feelings are easier to keep in proportion the next day.
   2. After a short night people are quicker to lose their temper and slower to shake off a setback.

Stages of the sleep cycle

1. Stage 1: drifting off
   1. Muscles relax and you are woken by the slightest noise, so you may not be sure you were asleep at all.
2. Stage 2: light sleep
   1. Heart rate and breathing slow, body temperature drops, and more of the night is spent here than in any other stage.
3. Stages 3 and 4: deep sleep
   1. Brain activity slows right down and you are very hard to wake, so being woken here leaves you groggy.
   2. Growth hormone is released in these stages, so physical repair is concentrated in deep sleep.
4. REM sleep
   1. The eyes dart about under the lids, the brain is nearly as active as it is when you are awake, and the muscles of the body stay limp.
   2. Most dreaming occurs in REM sleep, and someone woken out of REM will usually be able to report a dream.
5. One full loop through these stages takes roughly 90 minutes, and you repeat it four or five times a night.
6. Deep sleep fills the early cycles and REM periods get longer towards morning, so the dream you remember is usually one from just before you woke.

The pineal gland and melatonin

1. The pineal gland is a small gland buried deep in the brain, about the size of a grain of rice.
2. As light levels fall in the evening, the pineal gland secretes the hormone melatonin into the bloodstream.
3. Rising melatonin lowers alertness and body temperature, and that is the feeling of becoming sleepy.
4. Bright light in the morning switches melatonin release off, so alertness climbs again and you wake properly.
5. A bright screen held close to your face at 11pm slows that rise in melatonin, so sleep starts later than it otherwise would.
6. A clock deeper in the brain tracks the light and dark outside and tells the pineal gland when to start releasing melatonin.
7. A run of late nights under bright light shifts the timing of that release across several days.

Turning this into marks

1. When two benefits of sleep are asked for, make them genuinely separate rather than one benefit described twice.
2. Name the pineal gland and melatonin, because saying that the brain gets you ready for sleep does not secure the knowledge mark.
3. The mark most often lost is giving the length of the sleep cycle without saying that most dreaming happens in REM sleep.