What decides when you feel sleepy

1. Some of what decides when you sleep comes from inside your body, and some comes from the world around you.
2. The internal side is your body clock, which runs rhythms and triggers the release of a hormone.
3. The external side is the set of cues your surroundings give you, and the strongest of these is light.

Bodily rhythms that repeat about once a day

1. A circadian rhythm is a bodily rhythm whose cycle lasts about 24 hours, matching the length of a day.
   1. The sleep-wake cycle is the example to give: alert through the day, sleepy at night, over and over.
2. Strength: Siffre's cave study backs this up, because a man cut off from daylight for months went on sleeping and waking in cycles rather than stopping.
3. Weakness: the strongest evidence comes from single people in extreme settings, so you cannot be confident the same pattern holds for everyone.

Bodily rhythms that repeat several times a day

1. An ultradian rhythm is a bodily rhythm whose cycle is clearly shorter than a day, so it repeats several times within one.
   1. The sleep cycle is the example to give, looping you through the sleep stages and REM roughly every 90 minutes.
2. Strength: the cycle can be tracked directly by recording brain and muscle activity through the night, so the evidence is objective rather than a matter of opinion.
3. Weakness: cycle length varies between people and from night to night, so a single figure hides a lot of individual difference.

Hormones, the pineal gland and melatonin

1. The pineal gland is a small gland deep inside your brain.
   1. It responds to how much light is reaching your eyes, stepping up its output as the light fades.
2. Melatonin is the hormone the pineal gland releases into the bloodstream.
   1. Rising melatonin in the evening makes you feel drowsy and lets sleep begin, and falling melatonin in the morning helps you wake.
3. This is the link between the internal clock and the outside world, because a gland inside your head is being driven by light outside it.
4. Strength: it has practical value, because it explains why a bright screen late at night makes dropping off harder and suggests an obvious fix.
5. Weakness: it is reductionist, explaining sleep through one gland and one hormone while ignoring worry, noise, caffeine and habit.

Zeitgebers and the part played by light

1. A zeitgeber is a cue from outside your body that resets your internal clock.
2. Morning daylight holds the clock at roughly 24 hours, while evening darkness lets melatonin rise and sleep arrive.
3. Strength: it explains real problems, such as why night-shift workers sleeping in daylight and travellers crossing time zones both sleep badly until their cue and their clock line up again.
4. Weakness: light cannot be the whole story, because Siffre went on sleeping and waking in cycles with no daylight at all.

Turning this into marks

1. Give a cycle length for circadian and ultradian rhythms, then name the pineal gland, melatonin and light.
2. Name Siffre whenever you argue that the clock is internal.
3. The mark most often lost is covering only the internal or only the external side when the question names both.