Teenagers are well-known for routinely staying up late and then having a long lie-in. Sometimes this is put down to laziness or their transition from childhood into ‘Kevin the Teenager’.
However, there are biological reasons which help explain why teenagers are late to bed and late to rise, and why they might not actually be getting enough sleep to make them healthy, wealthy, and wise.
Sleep matters. A wealth of evidence supports the importance of sleep for our physical, mental, and emotional functioning. Insufficient sleep has been linked to problems with attention, creativity, memory and academic performance, as well as increased impulsivity, and difficulties with mood regulation.
Sleep: a double-act
Sleep is driven by two processes working in tandem: the circadian rhythm and the drive for sleep (sleep homeostat).
The circadian rhythm is a roughly 24-hour cycle of changes in physiology and behaviour, generated inside our bodies. These rhythms come from certain genes being turned on and off (called clock genes) in almost all cells throughout the body. The master clock, (the suprachiasmatic nucleus in the brain), coordinates the rhythms within these cellular clocks.
The second system involves a balancing (homeostatic) process. Put simply, the longer you have been awake, the greater the need for sleep will become. The drive for sleep is thought to stem from a build-up of various chemicals (in the brain) while we are awake. For example, adenosine is a building block of our body’s energy currency (ATP), so it builds up as a consequence of energy use in the brain. Adenosine inhibits wake-promoting neurons and stimulates sleep-promoting neurons. As we sleep, adenosine is cleared and sleep pressure reduces.
Sometimes the two systems oppose each other. For example, we would feel very sleepy mid-afternoon due to increasing sleep pressure if it were not for the circadian drive for wakefulness.
Sleep occurs when the circadian drive for wakefulness ends and the drive for sleep kicks in. Then when the ‘sleep debt’ you have accumulated during the day has been re-paid (and so sleep pressure is reduced), and the circadian drive for wakefulness is sufficiently strong, we wake up.
Why some people don’t like mornings
Some of us love to burn the midnight oil, while others schedule in regular morning runs. The technical term for your preferred sleep/wake timing is chronotype, what we colloquially refer to as being an owl or a lark. You are owl if your biological clock runs slower and a lark if it runs faster.
We don’t run on 24 hours
While our day runs on 24 hours, for most of us, our internal clocks don’t. Therefore, our internal clock needs to be adjusted daily by external time cues such as light intensity. The master biological clock receives light signals direct from cells in the retina, which are most sensitive to blue light.
Dusk triggers the production of melatonin (the ‘vampire hormone’). Although not a sleep-inducing hormone, melatonin serves as a cue for rest in humans, with levels increasing as we approach sleep and remaining high during the course of the night.
In contrast, dawn light suppresses melatonin levels. The circadian clock responds differently to light depending on the timing of exposure, so that morning light advances the clock (making us get up earlier) and evening light delays the clock (making us get up later the next day).
As well as playing a role in sleep regulation via the circadian clock, light affects how alert you feel. You may have noticed that bright light increases your alertness, so relatively bright light before bedtime will increase the likelihood that you will feel sleepier later.
Stay tuned for next week’s blog about how changes in these systems affect teenage sleep.