This article from New Scientist asks whether sleep walkers can be held morally responsible for acts committed while unconscious.  A different question is what effect a sleepwalker exception would have on crime.  Would it encourage sleepwalkers to commit more crimes, or on the extensive margin would it encourage more to people to take up sleepwalking?

I have no personal experience, but if sleepwalkers believe they are awake, then they would not be motivated by an exemption for sleepwalkers.  Of course at any moment in time all of us may be sleepwalking and assuming we all account for that small probability, a sleepwalker exemption would encourage more crime across the board.  This could be easily compensated by a small, uniform increase in penalties.

We shold worry that non-sleepwalkers will make use of the additional defense at trial (the “jammie defense?”)

Nightcap Nod: Mindhacks.