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Deals damage if the target is asleep. The user recovers half of the HP lost by the target, rounded up. Fails if the target is not under the sleep status.
There isn't much competitive value for this move, as it requires a very specific situation in order to work, that being the target needs to be asleep. As soon as you put a Pokemon to sleep (an already difficult task considering the low accuracy of most sleep inducing moves available) your opponent can simply switch out to make Dream Eater useless, and thanks to Sleep Clause, you can't put another one to sleep, making Dream Eater useless. Sleep is such a powerful status condition that most of the time it's better to use the guaranteed one sleep turn to get a frail sweeper onto the field for free or to use a set-up move of your own. Use Psychic instead.
Psychic has only a slightly lower Base Power and can be used at any time, not simply when the target is asleep. It also lowers Special 30% of the times.
Mega Drain is a much weaker, more reliable Grass-type equivalent.