As cows get closer to giving birth, their bodies start showing stronger daily patterns in stress hormones, pregnancy hormones, and mood-related chemicals — but only if they’re on a steady light schedule.

Before giving birth, cows have daily rhythms in body temperature and key hormones, but after birth, these daily patterns disappear, no matter if their light schedule was changed or not.

When cows are kept on a changing schedule of light and dark before giving birth, their bodies produce less serotonin and more melatonin, which might affect how their internal clocks work.

The daily changes in body temperature and hormones in pregnant cows aren’t controlled by just one clock — they’re shaped by at least two different timing systems working together.

When cows are exposed to shifting light schedules before birth, their daily body temperature and hormone patterns change — and those changes are connected to them carrying their calves longer than usual.