An internal circadian clock for people is normally slightly longer than 24 hours (24 hours, 11 minutes on average). Entrainment (in chronobiology) is the process of adjusting this internal circadian cycle to external 24-hour Earth rotation cycle.

For some people (including me), the internal circadian clock is longer and is closer to 25–26 hours, which makes entrainment even more important.

Factors that influence entrainment are called Zeitgebers (“synchronizers”)

  • Light: bright in the morning, dim in the evening.
  • Exercise: do exercise in the morning, light activity in the evening.
  • Food: stop eating a couple of hours before going to bed.
  • Coffee: no coffee in the evening. Coffee in the early morning is ok.