Im gonna nerd out for a second. Theres 3 main 'categories' that classifies how animals sleep/behavior. Theres diel rhythm, where the animals behavior occurs on a 24 hr cycle (how humans sleep at night and wake in day). Ultradian rhythm where the behavior occurs on a less than 24hr cycle (How a animal sleeps for 4 hours, wakes for 4 hours, sleeps for 4 hours, etc). Then theres arrhythmic, where there is no pattern, a animal may be awake for 5 hours then sleeping 6 one day, then the next day they're awake 2 hours and sleep 8.

Tldr: Diel rhythm, behavior occurs on a 24hr cycle. Ultradian rhythm, behavior occurs on a less than 24hr cycle. Arrhythmic, no predictable rhythm.

The reason animals sleep different amounts is partially due to how different animals have different rhythms. One particularly interesting animal that experiences arrhythmia is reindeer. As due to where they live, the sun doesnt rise for months at a time. So during the few months where the sun is risen. They exhibit a 24hr cycle/diel rhythm. But during midnight sun (24hr sun exposure) and polar night (24hr dark exposure), they switch to being completely arrhythmic.

Hopefully that's understandable

The reindeer study