I found out that I sometimes experience a-la daydreaming during the interview.
During this time I, instead of solving the problem, just sit and do nothing. Thinking about something else.
It happens, for example, if I am asked to solve a complicated problem. I start thinking about super-puper optimal way of solving it and i'm lost in my mind process. Like I think -- heh, it is a nice example of a general problem of the group of permutations Sn, than it reminds me about theorem from group theory, bla-bla and I'm lost for 5min.