A new employee and new students of the university are easy to fall this symptom. Mental condition that people cannot get used to be in the new environment is very related to it. Even if we have high motivation for new environment, some of us who cannot adopt the environment get it after the first week in May, you know, Golden Week. They get tired very much during GW, and lose mind that they want to go office or school.
To avoid this, we should refresh our mind by meeting our old friends or keeping regular life style in April. We also should not expect too much for our new environment. I think working off the stress is the best solution.