Zeitgeber

What it is

The word is German for “time-giver.” Coined by chronobiologist Jürgen Aschoff in the 1950s, it names anything in the environment that synchronises an organism’s internal clock to the outside world. The dominant zeitgeber for humans is light hitting the retina, but food timing, exercise, ambient temperature, and even social schedules are weaker secondary zeitgebers.

Why it matters

Without strong, consistent zeitgebers your internal clock free-runs to a period slightly longer than 24 hours, drifting later each day. Modern life supplies weak and contradictory cues: dim mornings indoors, bright artificial nights, irregular meal times. Strengthen the cues — get bright light in the first hour of the day, eat on a consistent schedule, dim your evenings — and most circadian-driven problems improve without any pharmaceutical intervention.