There are of course much more interesting ways you can annoy your wizards but this one is good because days often map pretty well to sessions. However, unless your wizard is super conservative with their spells she always runs out and ends up having nothing to do for the rest of the session. So it's not good, and actually bad.
(Maybe if your game is high level enough this stops being a problem but I'm pretty sure I've never gotten to the point where wizards start being OP in anything, ever, even Neverwinter Nights. The whole high-level you-all-own-castles-and-fight-Demogorgon thing is just a meaningless abstraction to me. Like finding Mew under the truck when you were eleven. Cool to think about but what, in practice, would even be the point? The parallels between epic levels and legendary Pokemon are pretty close actually.)
I've been reading through Patrick Stuart's blog from the beginning (to fill in time between grading student papers, of which I have many) and he provides a pretty neat rule to solve this problem. Basically if you want to cast a spell that you don't have prepared you can still do it, you just have to roll on the Terrible Magical Calamity table. This is obviously a Good Rule. (I think he got it from someone else who got it from someone else, etc etc., whatever.) I am going to change it a little though.
Here's your spell list or whatever. People like choosing spells! There are more in the Pathfinder SRD but honestly you don't want to give them too many. How it works is: for each sleep cycle you get you can prepare one spell. Sleep cycles are about ninety minutes. This means if you sleep for nine hours you can prepare six spells. You can cast each spell you've prepared a number of times equal to half your level (rounded up) minus its level. This obviously means that you can't prepare Level 3 Spells until you're level seven, or you can but there'd be no point.
You can also cast any other spell on the list at any point. However! If you haven't prepared a spell, or if you have but you've run out of castings, you have to roll on the Terrible Magical Calamity Table. The Terrible Magical Calamity table looks a little bit like this.
20: Spell works fine. A bit better than usual, even. (+1 to healing or damage or w/e).
19 - 17: Spell works fine. Life continues as normal.
16 - 14: Spell a bit shit. (-2 to healing or damage or w/e)
13-11: Spell a bit shit. Also, minor magical calamity (you start bleeding worms from your ears, your boyfriend is a ghost now, etc.)
10-8: Spell completely fails to work. Also, minor magical calamity.
7-5: Spell does exactly the opposite of what you wanted it to do, unless that would be good. Also, minor magical calamity.
4-2: Spell does exactly the opposite of what you wanted it to do. Also, two minor magical calamities.
1: Terrible magical calamity (the moon eats the sun and shits burning gold, etc). Spell works fine though
Maybe not exactly like that but you get the picture. You get -1 to your roll for every unprepared spell you have tried to cast since the last time you got at least one sleep cycle. You get -1 to your roll for each level the spell is above half your level (rounded up). So feel free to cast meteor swarm at level three, that sounds really funny. Maaaaaaaaybe you could add your INT bonus to this roll if you were, you know, a lame nerd.
You'll need a minor magical calamity table, obviously. Surely there's a billion of those. Here's one now:
- You start bleeding worms from your ears. The worms can talk. They have little spiked helmets. They hate the worms from your other ear. You must broker a peace between them or they will make your skull into a WW1 battlefield complete with trenches and mustard gas
- Your boyfriend (or best friend, or whoever's important in your life) is a ghost now. Only you can see them. Everyone else has forgotten they exist. They all think that you have gone crazy and that your brain is filled with false memories. 20% chance they are right
- Everything that is a specific shade of purple ceases to exist. The shade is one that has particular meaning to you, e.g. it was the colour of the flowers on your father's favourite apron or the first ooze you ever killed. Objects just a few shades off are unaffected
- Gravity is now reversed, but only for your blood. It would be a good idea to stand on your head. If you are cut you will fountain blood into the sky and it will look cool
- The closest urban area to you (including ones you happen to be standing in) now has a different system of government. If it was a monarchy it's now a democracy, if it was a democracy it's run by monks. It's about as nice a place to live as it was before. Only you and your friends notice anything different
- The next time you go to sleep a purple tiger will emerge from the nearest mirror. It will be about the size of the nearest mirror. It thinks it is the only living creature who knows how to do magic properly and to safeguard the security of magic it must kill everything else that tries to do it. It's the same shade of purple as in #3
- Game jumps ahead to d6 hours from now. Players have clearly done stuff during missing time but have no idea what it is. Everyone makes a Wisdom check to not have made at least one terrible mistake. Ideally, at bottom of dungeon holding sacred orb w/ legions of vengeful troglodytes between them and exit, but up to DM
- Climate cools by fraction of degree. Global, permanent. Harvests become poorer, winters come earlier, less evaporation means less rain means rivers become drier, probably. Long-term political ramifications
- New type of goblin invented. Will emerge from swamps en masse in 1d4 weeks to begin waylaying travellers, stealing babies, terrorizing populace, other normal goblin shit. Also have motivation specific to new type of goblin e.g. break all ceilings, build golems out of dung and nailclippings. If you die they all die. They know this
- Spell you tried to cast no longer exists. Only the insane and insomnomancers (see below) can ever cast it ever again. It is now twice as powerful and has unpredictable extra effects
One of these also happens if a wizard is woken up while she is halfway through a sleep cycle. Don't wake up wizards. Again, you could give them a save, if you were lame. You will note that this system provides a powerful incentive for wizards to use potent sleeping drugs to remain in a state of Ultimate Slumber for five, ten days at a time. This is very much by design.
They don't go into any kind of mysterious dream world where they have to track down spells through the spirit world and catch them in their teeth! I want to stress that. Mysterious dream worlds are lame.
Also some wizards will say "fuck it" and purposefully stay awake as long as possible. They are called "insomnomancers" and they are terrifying, probably. They gain more weird, perverse powers with every day they manage to stay awake, although I can't think of what those might be right now. They accumulate a few minor magic calamities that they learn to live with. They also acquire some of the following symptoms of sleep deprivation, copied directly from Wikipedia:
- Aching muscles
- Memory lapses
- False memory
- Hand tremors
- Malaise (a feeling of general discomfort or unease)
- Stye (horrible eye pimples)
- Periorbital puffiness (bags under the eyes)
- Nystagmus (rapid, rhythmic, involuntary eye movement, also a great wizard name)
- Temper tantrums
- Attention deficiency