Why you get sleepy: sleep pressure, your body clock and screens

Sleep isn't wasted time. While you sleep, your body grows and repairs itself, and your brain sorts and stores what you learned that day. New Zealand's health guidelines recommend 9 to 11 hours a night for children aged 5 to 13, and 8 to 10 hours for 14 to 17-year-olds. So why is it so hard to get there?

System one: sleep pressure

From the moment you wake up, a chemical called adenosine slowly builds up in your brain. The longer you are awake, the more there is, and the sleepier you feel. Scientists call this sleep pressure. When you sleep, the adenosine is cleared away, so you wake up with the pressure low again. Caffeine, found in coffee, cola and energy drinks, works by blocking adenosine, which is why it can keep you awake long after you drink it.

System two: your body clock

Deep in your brain is a tiny cluster of cells that acts as your body clock. It runs on a cycle of about 24 hours and is set mainly by light. In the morning, daylight tells it to wake you up. In the evening, as light fades, your body releases a hormone called melatonin, which tells the rest of your body that night is coming. Melatonin usually starts rising two to three hours before your normal bedtime.

Why puberty changes things

During puberty, the body clock shifts later. Many young people don't feel sleepy until later at night, but school still starts at the same time. That is biology, not laziness. It also means a regular wake-up time and morning daylight matter even more, because they help pull the clock back.

Light, screens and the brain

What helps

Dim the lights in the last hour before bed, charge your phone outside your bedroom, get outside in daylight in the morning, and keep bed and wake times about the same, even at weekends. Small changes that you can keep going are worth more than a perfect plan for three nights.