Oh yes, it happens to me all the time! One time I got home at 5:30, and sat down on my bed, shoes still on even, and woke up six hours later!!!!!!!! I had no intention of going to sleep when I sat down! I sleep a good 9 hours or so a night, going to bed exhausted ( but usually not stressed, there is a difference) and keen to wake up and get going in the morning. I think that this much mental activity IS tiring, and there is also a stress component of the PhD that is tiring as well.