when one is not tired or after one has rested ( usu. after a night's sleep)
when (while) one (one's mind) is fresh
with a clear headwhen (while) one's head is clear.
«Не получается у меня задача». - «Это потому что ты устал. Подумай над ней завтра, на свежую голову». "I can't figure out this problem." "That's because you're tired. Work on it tomorrow, when your mind is fresh."