The Periodic Universe.
This late 19th century mechanical planetarium, known as an orrery, illustrates planetary movements.

An Illuminated World.
This modern globe shows the natural cycle of day and night as the Earth rotates on its axis.

What do these actograms show?
The multi-crab "live" recordings of crab activity should theoretically match the one displayed above, which shows the activity of a single crab in a controlled laboratory setting. In the laboratory-generated actogram, the crab's locomotor activity can be seen to follow tidal rather than circadian rhythms. The circles indicate high tide, which occurs every 12.4 hours, while the bar at the top of the page indicates sunrise and sunset. To see if these live crabs are following tidal patterns, you will need to check their activity against the tide chart.

Fiddling while the Tides Rise — Uca minax.
These tiny fiddler crabs show tidal and circadian rhythms. Their exterior color changes according to a 24-hour schedule, whereas burrowing and other locomotor activities correlate with high and low tides. These tidally timed behaviors continue even when crabs are placed in a laboratory aquarium that has no tidal movement. These crabs, which are normally quite "shy," may not behave as expected in the presence of artificial light and visitors.

Lunatic Influence.
The ocean's daily tides are driven by shifting gravitational pulls that are created as the moon orbits the Earth.

Solar Synchrony.
For humans and most other organisms, light receptors are used to reset the clock, thus keeping the organism synchronized with its environment.

A Single-Celled Clock with a Glowing Reputation — Gonyaulax polyedra. Gonyaulax are single-celled free-swimming algae, that exhibit bioluminescence according to a circadian schedule. Bioluminescence is the abilty of living organisms to emit visible light. This circadian expression of bioluminescence is the result of the daily synthesis and destruction of two specialized proteins. The color image showing day and night fluorescence is for a single organism and shows the specific sites within the cell that emit light.

Seasonal Migrants — Sandhill Crane, Inca Tern, and Grey Seal. From building nests and reproducing to migrating, many animals follow circannual (yearly) rhythms. How do the migrating animals find their way, often across thousands of miles to precise locations? They rely on internal clocks and their position relative to the sun to determine their longitudinal (east-west) positions, and they may rely on their position relative to the stars or to the Earth's magnetic fields to determine their latitudinal (north-south) positions.

The Seasonal Fashions of Siberian Hamsters. The Siberian hamster, like many other mammals found in northern regions, has a light coat color in winter and a dark one in summer. The circadian clock in the hamster's brain registers progressive changes in the length of the night, as winter or summer approaches — and hormonally triggers the change in coat color. This is accomplished through the release of the hormone melatonin. As the length of the night changes, so too does the amount of melatonin released.

Credit 10

Yearly and Daily Cycles. In addition to circadian cycles, many animals also follow circannual or yearly cycles. One such cycle is hibernation, which occurs when daylight hours decrease. Bears are so-called opportunistic hibernators in that they will periodically arouse from the hibernating state. Even when animals are hibernating, however, their circadian systems remain operational.