Our bodies regularly release cortisol. Levels of this hormone in the body are highest in the morning. It lifts our mood and marks the phase of the day in which we are active. In the evening and towards night time, the cortisol level lowers again, reaching its trough between two and four in the morning.
Normally, we're asleep during these hours and as a result the low level does not have much of an impact. However, for people who lie away at night, mulling things over, the lack of cortisol has a negative outcome. The less cortisol is released, the lower their mood becomes. This means that thinking at night can no longer be considered positive, and so the thoughts that disturb our rhythm and bring us to mull things over need to be written down before we go to sleep.
If you look at these notes the next morning, when your cortisol level is high again, you will find that they seem much less stressful. In many cases, our negative assessment of a situation at night is put into perspective in the light of day.
Source: Schlafen für Aufgeweckte, Dr. med. Michael Feld, 3. Auflage 2015