What controls our sleep pattern?

Image of Dr Simon Kyle
By Dr Simon Kyle

Sleep is an automatic process and therefore out of our own direct, voluntary control. Whether awake or asleep we are at the mercy of two biological processes: Sleep Homeostasis, commonly known as (1) 'Sleep Pressure'; and (2) The Circadian Rhythm, otherwise known as the 'Body Clock'. These two processes work in harmony to promote good consolidated sleep at night.

Sleep pressure can be thought of as the brains pressure and need for sleep, becoming greater with increasing amounts of time spent awake. In this way, the pressure to sleep is directly related to the amount of time that we have been awake. For example, when we wake-up in the morning after a good night's sleep, we will have a very low sleep pressure or 'need to sleep'. As we continue throughout the day, sleep pressure will begin to accumulate (a bit like an hourglass egg-timer). At the end of a full day, at bedtime, we will have a great amount of pressure to sleep. By going to bed and having another good night's sleep, then sleep pressure will be reset for the start of the next day.

The Circadian Rhythm (Body clock) is an internally generated biological rhythm that allows a number of processes to rise and fall over the twenty four hour period. Commonly, its effects are mostly realized with jet lag when one travels through many different time zones rapidly. This is when the circadian rhythm is out of synchrony with the new environment and can take a number of days to go back to normal.

In a good sleeper, who is in-synch with the environment, the circadian rhythm will naturally rise in the early morning, promoting wakefulness and alertness – this is sometimes known as the alerting force. As the day continues, the circadian rhythm will promote wakefulness until it reaches a peak at about midday. After this time, the circadian rhythm will start to dip. This initial fall is known as the 'post lunch dip' (you may be familiar with greater feelings of sleepiness after lunch) and as a time for a siesta in other cultures. As we continue through the day, the circadian rhythm continues to fall and does not promote as much arousal as before. With the onset of bedtime and sleep, the circadian rhythm drops to the lowest level and helps to maintain sleep. In this way, sleep pressure is very high, while the alerting effect of our body clock is low, creating the optimal opportunity to sleep.

After this low point, the circadian rhythm will then rise again in anticipation for the next day. The body clock is difficult to manipulate and may be disrupted in a number of poor sleep conditions.

Filed under: Sleep science