Our urge to be asleep or awake is called our circadian rhythm. This has developed over millions of years and follows a cycle of approximately 24 hours. Given the entirely predictable phases of light and dark, determined by the rotation of Earth, it’s no surprise that evolution has used this day/night cycle as our body’s main reference point.
Our circadian rhythm follows the same pattern for everyone:
- When you wake in the morning your blood pressure and temperature rise.
- You stop producing melatonin and start producing serotonin, testosterone, adrenalin and cortisol.
- This leads to increased alertness, better coordination and faster reaction times.
- By early evening your body temperature and blood pressure are at their highest.
- Later in the evening you stop producing serotonin, testosterone, adrenalin and cortisol, and start producing sleep-inducing melatonin.
- You fall asleep, and your blood pressure and temperature fall.
- You reach your deepest sleep in the earliest part of the night and lowest temperature and blood pressure in the middle of the night.
The production of melatonin, which helps us feel sleepy, is triggered by a reduction in light entering our eyes, as detected by the Suprachiasmatic Nucleus in our brain. For millions of years this process was organically triggered by the Earth’s natural light and dark cycles, but for a fraction of our species' existence unnatural light has extended our day. In addition, we now have jobs that require unusual hours, we rush our food, suppress our urge to use the toilet and (just when we should be winding down ready to sleep) we watch TV, check our emails, or engage in social media. All of which override our body's natural circadian rhythm.