In The Home Brewer's Answer Book a similar question was asked on pg 391. I won't retype the question or the answer here as they are both quite long. However, the answer is somewhat interesting as a lot of things are addressed, such as the dryness of the beer and increased fermentability, using an overnight mash for souring the beer on purpose etc. Long story short though, it is okay to do this but you really need a good insulated mash tun that won't lose very much of its temp. You could start your mash, got to bed and sparge the next morning. Your efficiency will probably rise and from what I can tell from his answer, light bodied, higher gravity beers may actually benefit from this practice.