Night-night: The hormone adenosine suppresses brain cell activity, leading to an increased feeling of drowsiness the longer a person has been awake. During sleep, the adenosine is broken down. Caffeine binds to adenosine receptors, so it can help people stay awake. Photo: Aja Koska
The urge to sleep begins early in the day, from the moment that you first wake up and get out of bed. Shortly before you wake, your body releases a surge of hormones, including the stress hormone cortisol, which prepares you for the day. But waking also triggers the release of a chemical in the brain called adenosine.
Adenosine binds to receptors in your brain, slowing down brain activity. This suppression of brain cell activity is what causes the feeling of drowsiness.
The longer you are awake, the higher your adenosine levels rise. The higher the adenosine levels, the sleepier you get. Then, when you finally go to bed and sleep, the adenosine is broken up and disposed of.
If you are feeling sleepy and want to stay awake, you can, temporarily, block the effects of adenosine by consuming the world’s most popular psychoactive drug, caffeine. Caffeine binds to sleep-inducing receptors in your brain that would otherwise be occupied by adenosine — that’s why it wakes you up.
As you may have noticed, caffeine has very different effects on different people. There are some people, like my wife, who are really sensitive to caffeine. She only needs one cup to wake her up. Caffeine also hangs around in her system for much longer than it does in mine.
The reason you have to keep topping up your caffeine is that it is constantly being broken down by your liver. The average half-life of caffeine is around five hours, which means that, if you have a cup of coffee at 6pm, half is still running around your system at 11pm and a quarter is still there at 4am.
If you are sensitive to caffeine, you will still be feeling the effects of an afternoon cup of coffee in the middle of the night, lying awake and wondering why you can’t sleep.
But quoting average figures is misleading because some people break down caffeine much faster than others. The “average” half-life of caffeine may be five hours, but the range is actually 1.5 hours to nine hours.
If you are someone whose body breaks down caffeine especially slowly, you will still be feeling the effects of your morning cup of tea or coffee in the middle of the night. But, if you are naturally able to break down caffeine rapidly, then you can drink coffee in the evening without it disturbing your sleep.
Various things affect the half-life of caffeine in your body, including your gender, age, weight and any medication you are on. Taking the oral contraceptive pill will dramatically slow your liver’s ability to break it down. The biggest factor, however, is your personal genetics.
As well as the build-up of adenosine, the other major driver of sleep is your circadian clock. Deep in your brain there is a small group of cells called the suprachiasmatic nucleus (SCN). If I were to drill a hole between your eyebrows and carry on drilling into your brain until I hit the hypothalamus, and if I were then to stick an electrode down that hole, you would actually be able to hear the clock tick.
Oddly enough, your circadian clock doesn’t follow a day that is exactly 24 hours long. Some people’s clock runs fast, others’ runs slow. If you have a fast clock, then that means you are a lark — you like to get up early.
If you have a slow clock then you are an owl, someone who likes to stay up late.
The reason you don’t go completely out of whack is that your internal clock is reset every day by light.
Rays of light from the sun hit receptors in the back of your eyes that have nothing to do with vision but instead are linked to the SCN. The SCN then sends out signals to other parts of your body, including your guts, letting them know that a new day has begun, and it is time to get moving. It’s a bit like shouting at the kids, “Wake up, breakfast will be on the table in 20 minutes.”
Similarly, just as we like to have a warm house when we get up, the SCN raises your core body temperature before you wake, so you are ready to get going. It will also, in the early hours of the morning, switch off the production of melatonin — a hormone connected to your brain clock that is released when it gets dark to help tip you into sleep — and switch on the release of the stress hormone cortisol.
What makes things a bit more complicated is the fact that each of our organs has its own clock, which is linked to the main clock but not necessarily driven by it. The clock in your liver, for example, is not reset by light but by when you eat. This is important, because when your biological clocks get out of sync with the outside world, you are in trouble. Not only will you struggle to sleep, but you will get hungry, have problems controlling your blood sugar levels, feel run down and tired, and find it hard to concentrate.
Fortunately, you can retune your clocks and get your body back into sync with some relatively straightforward tweaks. These involve making sure you eat the right food, at the right time, and that you get exposure to strong enough light, again at the right time. That is what this book will help you do. Fix the clocks. Fast.