> I definitely want to make sure mage characters don't try to take a nap
>each spell they cast. That's where the not-so-random encounter table comes

You can also distinguish between being tired, fatigued, and exhausted.
Casting spells may make you weak and fatigued, but the recovery may not be
sleep, but serene meditation. If so, sleeping may not restore no more than
sitting/resting (4/hr). Substitute serene meditation for the 8/hr recovery
and rule that it can only be done in a place of assured security. The
wizards sanctum, a trusted ally's fortress (assumning its not too spartan),
a druid's glen, a friendly temple. Not local inns, a barn, caves, or any
other place a party might want to post a watch.

In my campaign sleep is only allowed when players are tired (been awake for
a day).
Exhaustion rules are used for physical activity (running, fighting &c) and
Fatigue would be used for channeling.

Kenneth Gauck