Hey there,

I've heard of it before, but in all the years that I've been lucid dreaming, never experienced it myself. On the contrary, I often feel more rested, rather then less (though I think that's really just cause I feel more satisfied with my night in general).

One explanation could be that your friend, in trying to achieve lucidity, is messing with his sleep rythme. Waking up in the middle of the night, staying up for half an hour there, tossing and turning, these things can all lead to one having less sleep and less restful sleep (not necessarily the same thing).. which will make him more tired in the morning.

Another possibility is the reverse. It's a fairly well known phenomena that it's easier to become lucid when you sleep less deep. It could be that there's something in the nights that he reports that makes him unable to sleep quite as well as he normally does. This makes him sleep lighter, and therefor get lucid dreams. But in that case it's not the lucid dreams that are the cause of his tiredness, they're a side-effect of it.

Then again, it could be genuinly possible. It's always hard to say, the body and the mind are unpredictable things. What's true for one person isn't necessarily true for another. But I think just about any lucid dreamer will tell you though, getting good consistent lucid nights is always a matter of finding the right balance. In your friend case, maybe he just hasn't found that right balance yet.

Hope that helps,

-Redrivertears-