How do we know when it’s time to eat, sleep and wake?

Aims:

• To introduce the concept of circadian rhythms, their physiological effects and individual variances
• To introduce the key hormones involved in regulating our body clock

Curriculum links:

• Human reproduction
• Coordination and control
• Health: non-communicable diseases

Take home messages:

• Hormones regulate our body clocks
• Our body clock tells us when it’s time to eat, sleep and wake up
• Key hormones:
  • Cortisol levels build up in our body while we sleep and signals to us that it’s time to wake up. Have you ever notice that you wake up every morning at the same time? Or just before your alarm goes off? This is because of your hormones!
  • Ghrelin is produced by the stomach; it promotes appetite, uptake of food and storage of fat. It tells our brain that we’re hungry and it’s time for lunch!
  • Leptin is produced by fat cells when we eat and signals to the brain that we’re full. Leptin deficiency leads to obesity as people don’t know when they’re full and should stop eating. This shows that obesity has a genetic basis and isn’t always due to life style.
  • Melatonin is higher in the evening and at night when we are asleep. It makes us feel tired and reduces our body temperature – helping us fall asleep. Disrupted pattern of melatonin is why we suffer from jet lag when we travel to a different time zone.

Discussion points:

• How do our bodies know it is time to sleep and time to wake up?
  • Endocrine organs release hormones in response to cues like light levels, blood sugar levels. The hormones act as messaging molecules that act on other organs in the body.
  • Melatonin is a good example – more is released from the pineal gland in the brain when there is less light coming into the eye and increasing melatonin levels have effects on the eyes and how heavy they feel, providing our body with a with a physical signal that it is time to sleep.
• Humans and other species have biological clocks throughout the body, all controlled by a ‘master-clock’ in the brain - this varies between individuals – some people are ‘night owls’ and others are ‘morning larks’ – we know that there is a genetic basis for these differences
• Scientists have found our body clock preferences change through life – the very young and the very old tend to wake up earlier, teenagers’ body clocks tend to make them wake up later – there have been debates about whether secondary schools should start later in the morning so students feel more alert.