I've seen some interesting arguments that 'sleep' is actually the default state of living organisms, and being 'awake' is the temporary altered state evolved on top of that.
FTA
And an interesting comment of someone on FB:
Hydra
Hydra have a diffuse neural system
But no brain to speak of
A simple creature echoing older, more primitive forms
They doze off, so we've found
Using familiar chemistry
To slumber every four hours
When I sleep I descend like a bathysphere Down through layers of earlier incarnations
Dipping below the surface
Leaving the airy light of consciousness behind
To enter the involuntary dream world of Freud and Jung
Deeper still then dreams themselves disperse
Slipping away like darting fish
Leaving me reptilian, autonomic, just breath and pulse
But there's further to go - into the abyssal dark
To reach my fundamental hardware
Where I am the hydra
Supporting evidence: horses sleep 2 hours, cats sleep 16, and even humans vary widely in how much sleep they need. That's hard to explain if all of it is fundamentally necessary, but easy to explain if the precise length is irrelevant for biological purposes and that the length was determined by outside factors e.g. how much this species stands to gain from remaining in a low-power state.
Edit: I also think that this is why you get tired when you sleep too much, your subconsciousness will run exhausting mental training when your consciousness is asleep so sleeping when your subconsciousness is alert makes you more tired.
It does seem rather coincidental that we sleep for 7-9 hours, which is the majority of the dark time, during which wakefulness would only expend energy with little gain (thinking of apes instead of modern humans)