A for adenosine. B for biological clock. C for cycles and conditions.

At MUUN, we spend an extraordinary amount of time talking about what makes and allows good sleep. Magnesium, mouth tape, a good mattress, melatonin, eight hours, no screens after 9PM. Some of these things matter a lot, but to really understand why, it helps to start with the biology behind it all. 

I always use a very simple framework, I call it the ABC of sleep. A stands for adenosine: the pressure that makes us sleepy. B stands for the biological clock: the timing that tells our body when sleep should happen. C stands for cycles and conditions: what happens once we are asleep, and the environment that allows sleep to continue.

A stands for adenosine

Imagine waking up in the morning with an almost empty hourglass, from the moment we wake, the sand begins to fall. That sand is a useful way of thinking about adenosine. Adenosine is a naturally occurring substance involved in energy metabolism in the body and brain, during the hours that we are awake, adenosine gradually accumulates and contributes to what sleep scientists call sleep pressure, the steadily growing need for sleep that builds the longer we stay awake.

By the evening, after sixteen hours (or so) of wakefulness, the hourglass is full, and when we finally sleep, the brain clears the pressure again, so that we wake the next morning with the glass almost empty and the whole process ready to begin once more.

What this means in daily life

The hourglass explains several things most of us have experienced without quite knowing why, as a practical understanding here are a few examples: 

A long/late nap empties part of the glass. Sleep of any kind releases pressure, which is why a generous nap at five in the afternoon can leave us lying awake at eleven with too little sand to carry us into sleep.

A bad night tends to correct itself. After poor sleep the pressure is higher the following evening, and the body usually responds with deeper sleep. Going to bed much earlier to compensate rarely helps, because the glass is not yet full.

The conclusion is reassuring. We build tonight's sleep during the day, simply by being awake and active for long enough.

B stands for biological clock 

If adenosine was the whole story, we would grow steadily sleepier from morning to night, and most of us know that this is not how a day feels like. We dip after lunch, recover in the late afternoon and are often surprisingly alert at nine in the evening, even after a short night. The reason is the second system, the biological clock.

Deep in the brain sits a small cluster of nerve cells, the suprachiasmatic nucleus, which keeps time for the entire body. It runs on a cycle of roughly, but not exactly, twenty-four hours. 

The correction comes from light and darkness.

Light reaching the eyes in the morning tells the clock that the day has begun, and darkness in the evening allows the brain to release melatonin, the hormone that signals biological night. Melatonin is often described as a sleep hormone, but it is more accurately a darkness hormone. It does not knock us out, it just tells the body what time it is.

Two systems surrounding one night

Sleep researchers have described the interplay between A and B for more than forty years in what is known as the two-process model, in which sleep pressure and the biological clock together determine when we sleep and how deeply. During the day, the clock sends an alert signal that grows stronger as sleep pressure builds, which is how we manage to stay awake and reasonably pleasant for sixteen hours. 

Sleep comes easily when the two are aligned, with pressure high at the moment the clock opens the gate.

What this means in daily life?

Morning light is the strongest signal we can give our biological clock. Daylight shortly after waking, preferably outdoors, anchors the rhythm for the rest of the day. On a grey November morning in Copenhagen the sky is still many times brighter than a kitchen lamp. Evenings should be dimmer than we tend to keep them in light; lower, warmer and fewer lights in the last hours before bed give melatonin room to rise.

C stands for cycles and conditions

A and B bring us to the edge of sleep. C describes what happens once we are there, and what allows it to continue during the night.

Cycles

Sleep is not one long, even state. Through the night we move through a repeating sequence of stages, from light sleep into deep sleep and on to REM sleep, the stage in which most vivid dreaming takes place. One full cycle takes around ninety minutes, and a normal night contains four to six of them.

The cycles are not identical. Deep sleep is concentrated in the first part of the night, when sleep pressure is highest, while the REM periods grow longer towards morning. A night is therefore front-loaded with deep sleep and finished with REM, which is one reason why cutting sleep short at either end costs us something different.

Between cycles, sleep becomes lighter, and most of us surface briefly several times a night. This is normal. A brief awakening at three in the morning is usually not a sign that something is wrong. It is the seam between two cycles, and whether we slip back into sleep depends a great deal on the conditions around us.

Conditions

Temperature. Falling asleep goes hand in hand with a drop in core body temperature, and the body needs to be able to release heat to stay asleep. A cool room helps, but there is no single magic number, but a good rule of thumb is roughly around 18-20 degrees celsius. 

Darkness. Light is the clock's signal for day and alertness, also at night. A properly dark bedroom protects both melatonin and the last, lighter cycles in the early morning.

When A, B and C work together

A good night is rarely the result of one clever intervention. It is what happens when the three systems meet. Enough pressure, at the right time, in a body and a room that allow sleep to continue. A bad night, in turn, can usually be traced to one of them, and the next time sleep goes wrong it is worth asking three simple questions before reaching for a solution.

A. Was I tired enough? Did I sleep late, nap long, sit still all day or drink coffee in the afternoon?

B. Was it the right time? Have my bedtimes and mornings been drifting, did I see daylight today, and how bright was my evening?

C. Was I allowed to stay asleep? Was the room too warm, too light or too loud, was I comfortable, and was there wine with dinner?

The answers will not solve everything. Persistent sleep problems have many causes, and when poor sleep lasts for weeks and affects the day, it deserves a conversation and some attention.