Macbook Air goes to sleep after waking up.

Hello all.

I have a curious problem with my MBA.

When powered on I will close the lid to put it to sleep. After a while I come back and open the lid to start doing tasks again.

I enter my password, do a few things and the thing falls back asleep. I have to move the mouse a few times to wake it back up.

Has anyone else had this problem?

My MBA is a 2012 model with Core i7, 8GB RAM, and a 256GB SSD.

Thanks.

I re-installed ML and that seems to have fixed the problem.