Pelorian Crusader

[FireSpark]

how does a caster with levels in this class prepare spells if they cannot sleep?

Because caster don't have to sleep, simply rest. And by resting, it usually means doing nothing strenous. If all a crusader is doing for eight hours is liesurely walking down the road, then I'd say that that qualifies as rest. After all, as mentioned above, Warforged don't sleep and they can be spellcasters. (Although the concept of rest does seem somewhat counter-intuitive to this class.)