If you're playing a divine spellcaster, check with your DM on this before picking an odd time to prepare your spells (like noon or sundown).
According to
this:
SRD said:
A divine spellcaster chooses and prepares spells ahead of time, just as a wizard does. However, a divine spellcaster does not require a period of rest to prepare spells. Instead, the character chooses a particular part of the day to pray and receive spells. The time is usually associated with some daily event. If some event prevents a character from praying at the proper time, he must do so as soon as possible. If the character does not stop to pray for spells at the first opportunity, he must wait until the next day to prepare spells.
SRD said:
A divine spellcaster does not have to prepare all his spells at once. However, the character’s mind is considered fresh only during his or her first daily spell preparation, so a divine spellcaster cannot fill a slot that is empty because he or she has cast a spell or abandoned a previously prepared spell.
SRD said:
As with arcane spells, at the time of preparation any spells cast within the previous 8 hours count against the number of spells that can be prepared.
To sum up:
Let's say you've got twelve spell slots (the levels aren't important for this). You prepare your spells at noon. The party wakes up in the morning, and heads out to the dungeon. You've got your full selection of spells, not having needed to use any the day before.
At 10 am, the group enters the dungeon and has a couple encounters. In defeating the enemy and mending the group afterward, you use up six of your spells. You have no other significant encounters in the next two hours.
When noon arrives, you demand that the party stops so that you can pray. When you do so, however, you can't prepare spells in any of the slots you used two hours ago; all you can do is abandon the spells you still have to replace them with other spells.
At 4 pm, you have another set of encounters, requiring you to use up all the spells you have remaining.
At 6 pm, the spell slots you used earlier in the day become available again.
Here is the tricky part you'll want to talk to your DM about. According to the description of a cleric's spells (in the class entry), the number of spells listed is a daily limit. Most DMs consider one 'day' to be from sunrise to sunrise. The second quote above states that spells previously cast can't be used in a second 'prayer session'.
By this interpretation, you have no spells at all until NOON the next day.
In my games, I allow divine spellcasters to make use of the rule that allows you to do a partial spell preparation, intentionally avoiding those spell slots that were used in the 8 hours prior; this way, when the 8-hour 'statute of limitations' ends, they can do another preparation session to refill those slots. Essentially, I count the 'day' part of 'spells per day' as beginning at the time the cleric normally does his prayers and spell-preparation.
Ignoring my ruling, let's make the above example worse; let's say you have nothing happen at 4 pm, and instead have more encounters the next morning at 10 am again. When noon rolls around, you can only prepare the spell slots that were unavailable the previous day, because the other six slots were recently used.
This means that any divine spellcasters, if you take an overly-strict interpretation of the rules, will almost always be at less-than-full ability on spells unless his preparation time is first thing in the morning, alongside the arcanists. One or two sessions of bad timing, and his spell selection is totally screwed until the group can take about two days off with no spellcasting on his part.