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I just glanced at some of the FAQs and the boards and most of them say that the spell "Sleep", while extremely powerful and useful in Pool of Radiance is useless in this game. That is not entirely true. Sleep eventually does become useless by the end of the game, however it is actually pretty useful in Tilverton. Sleep is extremely effective in the bar brawl and the big fight in the Thieves' Guild. Also all the small battles before reaching the sewers against the thieves, monkeys, and dogs. It starts getting less effective in the sewers, but can work on alligators, ogres, and trolls. It also has a chance of putting a single Fire Knife to sleep, a poor man's Hold Person (but it's a level 1 spell instead of a level 3 spell). Once you leave Tilverton on the other hand, Sleep does indeed become mostly useless. I think it might be effective in the bonus battles where you encounter bugbears and wargs when you search the areas to the northwest, but that's about it.
TL;DR? Sleep is useful in the beginning, up until you leave Tilverton, although mostly just in the town itself and the Thieves' Guild. You should stop memorizing it when you reach the Fire Knives hideout and expend any memorized copies by the end of the showdown with the Fire Knives leader and the Princess. Blow them all in that battle as a poor man's Hold Person.