lmorovan,
The books of Matthew and Mark do not record that they were sleeping.
According to Matthew 17:2-4, Peter was able to talk to Jesus during the Transfiguration.
According to Mark 9:2-5, again, Peter spoke to Jesus during the Transfiguration.
According to Luke 9:29-33, Luke's account is that the disciple were asleep, and then awoke. After they awoke, Peter spoke to Jesus.
There is no discrepency here. It is the same situation as Jesus' words on the cross. While different books emphasize different details, that is not to say that they contradict each other.
And who knows why God chose to have Luke add that detail? It very well could be that since Luke was a doctor, he would pay more attention to physiological details, such as the state of mind of the disciples. Because the book of Luke adds the detail that they were asleep and then awake is not evidence of contradiction.
Had the accounts in Matthew and Mark stated that they were constantly awake for the whole experience, I would understand your question. But it does not. Both of those accounts state that Peter was lucid enough to ask Jesus a question. When the disciples awoke in Luke's account, Peter was again lucid enough to ask Jesus the quesiton.
These are not contradictory, rather complimentary.
By His Grace,
killianpsi