Night Time is the right time for sleep? Are you kidding??

[Dirk Gross]

I think in shorter scenarios, like 2-3 days (20-30 turns), there's not much of a disincentive to fight all night. But, for campaign games covering many days to over a week, you'll find it beneficial to withdraw damaged and fatigued formations for rest and refit. If you keep pushing on offense in the long battles, you'll become so ineffective that your attacks will stall and be ripe for a counter-attack.