Most advice about sleep focuses on habits. Go to bed earlier. Put your phone away. Try harder to switch off.

But sleep is not simply a habit. It is the result of two biological systems that continue working whether you pay attention to them or not.

The first is your sleep drive. The pressure to sleep builds for as long as you are awake. The longer you stay awake, the stronger it becomes. It works a little like hunger: it accumulates while you are awake and decreases when you sleep.

The research behind this program uses a helpful way of thinking about sleep: you can spend it.

Imagine that your body needs seven hours of sleep. If you take a two-hour nap, you have already spent two of those hours. That leaves less sleep drive available for the following night.

Remember this idea. It returns in Lesson 4, where it helps explain why a nap can make it harder to sleep that night.

The second system is your body clock. Independently of how long you have been awake, your body follows a cycle of roughly 24 hours. This cycle influences when you are naturally prepared to sleep and when you are more likely to feel alert.

That is why you can feel exhausted at 3 p.m. and wide awake at 11 p.m. It does not mean that something has gone wrong. Your sleep drive and body clock are simply pulling in different directions.

Your body clock is influenced more by when you wake up than by when you go to bed. This is why one of the rules in Unit 2 focuses on your mornings and asks you to keep the same wake-up time on Saturday as on Tuesday.

Almost everything in this program works with one of these two systems. It does not rely on willpower or on creating a perfect bedroom.

This also explains the hidden cost of lying awake in bed. Your sleep drive continues to build whether you are in bed or somewhere else, so those hours are not lost in that sense.

The real cost is what your body learns. Every hour spent awake in bed strengthens the connection between your bed and being awake. You may also drift into light sleep without noticing. Even light sleep can reduce some of the sleep drive you may need later in the night.

This may sound like a small detail, but it is the mechanism behind much of Unit 2. It explains why the central instruction in that unit is: get out of bed when you cannot sleep.

Start your sleep diary tonight. Complete it tomorrow morning based on the previous night. It takes about a minute, estimates are enough and nobody will see it except you.