Right. Clerics need no specific amount of time, but they can only restore their spells at a specified time- and no earlier (later is possible, but no earlier). Both methods have their drawbacks & advantages.
This does not mean a cleric can go without rest- there are fatigue rules. But getting only, say 6 hours of sleep, instead of 8 isn't going to hurt them alny. OTOH, arcane spellcasters can get their spells bac at any time-but they need a full 8 hours.
Doing tricks like that isn't fair- except once in a great while. I had one party that figured there'd be at most two randon encounters per day of overland travel, and then they'd get to rest. Thus they really unloaded on each wilderness encounter. So I had them get attacked a few more, time, and also not get to rest. That taught them to not simply unload every freaken spell on some random encounter- and allowed the fighters & such to have more fun. But don't do this very often- only when they start taking things for granted.