system
Given the following post, respond as if you're commenting on Reddit:
human
How 'complex' does life have to get to require sleep?

I don't think single celled organisms 'sleep', and know that humans, mammals, birds etc do. 

What's in between? Is there a species that "kind of sleeps" but maybe is on the verge of needing it or not?
gpt
The closest borderline cases are probably animals very extremely short life-cycles, but even then there's evidence of sleep-like states. In general, the presence of a nervous system is likely all that is required.