Ultradian Rhythms are periodic the body goes through multiple times per day. These occur in many ways across many body systems.
There are some folks (including the makers of Endel, which as it turns out is based around ultradian rhythms) who suggest aligning your schedule and productivity around Ultradian rhythms. A common suggestion seems to be working in blocks of 90 minutes, with 20 minute breaks separating them.
I’ve seen sources that define it very loosely, saying things like “blinking” are also Ultradian Rhythms.
Source
- Huberman Labs episode 1
- Some googling