I wake at 3am and can't get back to sleep

Sleep isn't uniform. In the second half of the night, deep sleep thins out and REM predominates, so the waking threshold is naturally lower. Everyone wakes briefly several times — the difference is what happens in the next thirty seconds.

Cortisol begins rising in the hours before waking, as part of the normal circadian rhythm. In an already activated system, that rise is enough to cross the threshold. You wake, and the mind switches on immediately onto the most loaded subject — money, work, an unresolved conversation.

The second layer is learned. After a few weeks, the body associates the bed with alertness, and the anticipation ("I'll wake up again") becomes a trigger in itself. This is where insomnia turns into a problem separate from whatever produced it.

Don't stay in bed awake for more than 20 minutes. Get up, go to another room, do something boring in low light, return when sleepy. It sounds wrong and it is the most effective component of CBT for insomnia (CBT-I) — it breaks the association between the bed and alertness.

A fixed waking time, including at weekends. It anchors the rhythm more strongly than bedtime does. Saturday catch-up sustains the problem.

A worry window, during the day. Twenty minutes at a set hour for whatever is churning. When the thought arrives at 3am you postpone it to tomorrow's window — it's a deferral, not a ban, which is why it works.

Natural light in the first hour after waking. It regulates the circadian rhythm more effectively than anything else available for free.

Don't look at the clock. The arithmetic of "four hours left" adds activation exactly when you need the opposite.