The biological clock.

It exists, hides in the brain and it makes one feel great or miserable. It's in charge of the habits of and organism. Habits if it comes to sleeping, eating, etc. Next to this central clock each bodycell has it's own biological clock too.

It's the biological clock telling us how we feel.
We all heard of, some even experienced it, the jetlag. Flying to a different time zone confuses the brain and this is the reason why one wants to sleep at daytime, eat in the middle of the night or is not able to sleep. To some it's really hard to reset their biological clock.
An other example is the Summer-Winter time which is used in some European countries.
The idea behind this was to save energy (use daylight more economically). In reality, the one hour difference brings nothing except for messed up brains. Especially sensitive people, and those who already have a hard time dealing with the fact their biorhythm differs from what the working society asks from them, suffer from it.

The group of people with sleeping disorder increases.
More and more seem to suffer from a lack of sleep. Many tips and tricks are given. Tips like how to relax, not to focus on not being able to sleep, how to avoid stress, better not to consume a hard to digest meal, etc.
At times it helps just like reading a boring book, listening to a speech, drinking hot milk, or avoiding to watch a movie, work on a pc/phone during the last hour before bedtime but it's rarely the solution.

What causes sleepless nights?
Besides of worries, sorrows and being overexcited it's the biological clock.
Since humans changed their lifestyle, switch on the light, watch tv (more light) and the difference between day and night became smaller (not enough light during the day and too much in the evening and night) the biological clock is messed up and stays messed up.

A good night rest.
Tom Wehr, an American psychiatrist, tested sleeping behavior. Those who joined his experiment lived their normal life during the day but there was no light during the evenings/nights. Two things happened. The most obvious one was they all went to bed earlier but their sleep pattern changed as well. Not one person slept through the whole night. The new pattern was four hours sleep, about two hours awake followed by four hours sleep.

If this is the natural sleep pattern it explains to me why I frequently wake up around 2-3 am and fall asleep right before I need to wake up. My biological clock is upset which upsets me but it is good to know it's quite normal to wake up and not being able to sleep for some hours a night.