Module 2 in 10 slides

Your two sleep switches

Module 2 of Sleep, Decoded

Why you get sleepy at night, why you wake up in the morning, and the two everyday things that push on both.

Switch 1: the body clock

A large group of nerve cells deep in the brain, the SCN, is your master clock. It runs on a cycle of about 24 hours and helps make you sleepy at night and awake in the day.

Switch 2: sleep pressure

The longer you are awake, the stronger your drive to sleep gets. Sleep releases it. One chemical that helps make you feel sleepy is called adenosine.

Both switches, one night

Light sets the clock

Light and dark have the biggest influence on the body clock. Light soon after you wake tends to move it earlier; light late in the evening tends to move it later.

Evening light has a cost

In a lab study, ordinary room light in the hours before bed shortened the time melatonin (a darkness hormone) was present by about 90 minutes, compared with dim light.

Caffeine blocks the signal

Caffeine works by blocking the receptors that adenosine acts on. You feel less sleepy, but it is not a substitute for sleep.

6 hours is not too cautious

In a small study at home, a large dose of caffeine taken 6 hours before bed still cut sleep, as measured by a sleep monitor. The NHS advises no tea or coffee for at least 6 hours before bed.

Lark, owl or in between

Your chronotype shifts with age. Use it to plan when you do things, not as a reason to give up your anchor time.

Your jobs this module

1. Get daylight within an hour of waking.
2. Set your caffeine cut-off with the calculator.
3. Keep the diary going and look for a weekend shift.