Circadian Rhythm
Circadian rhythm is the body clock that tells an animal when to sleep, wake up, and eat.
A kitchen timer set to go off at the same time each day works like the body clock. Each morning the timer ticks forward and triggers an alarm that wakes the body up. Each evening it counts down again and signals the body to slow down and rest. The timer runs on its own — it does not need you to reset it each day.
Explaining circadian rhythm by grade level
A hamster sleeps most of the day and runs on its wheel at night. Its body has a built-in clock that says when to rest and when to play. If you change the lights, the hamster gets mixed up for a while. Its body clock needs time to match the new light.
Projects that explore circadian rhythm
The body clock makes animals more alert at certain hours. Hamsters seem sleepy at some hours and wide awake at others. Running a hamster through a maze at four different times over three days reveals its peak alertness.
Artificial light can reset an animal's body clock and shift when it sleeps and wakes. Hamsters normally stay active at night and sleep during the day. Giving one hamster artificial light during its usual dark period tests whether light can flip that pattern.
When a light cycle is disrupted, sleep and activity patterns take time to return to normal. To measure this recovery, you split ten hamsters into two groups and track their time on exercise wheels. After two days on the same schedule, one group gets a sudden shift in their light-dark cycle while the other stays on the original schedule. The shifted hamsters drop from about 150 minutes of wheel time to under 40 minutes. It takes roughly a week before they return to normal activity levels.
Social contact may help an animal's internal timer — a biological clock that follows day-night cycles — reset after a disruption. You house six hamsters in four cages: two cages hold single hamsters and two hold pairs. All follow a 12-hours-on, 12-hours-off light schedule for five days, then you shift the schedule by one hour for half the cages and track exercise wheel use over the next five days. Paired hamsters adapted to the new schedule more quickly. Solo hamsters took longer to return to normal activity levels.
Animals can learn daily feeding patterns as part of their body clock. Fish may follow the same kind of schedule that makes people feel hungry at set hours. Feed fish at the same time each day, then switch. Track how long the fish takes to adjust.