So Rintarou travels back in time by transfering his memories. Then a day passes and he transfers his memories back again. Another day passes and he transfers his memories again. And so on. Many days have passed but he returns to a body which recently slept (one day ago probably) but his mind havent slept for many days. Is it supposed he's a day tired or dozens of days tired? How does the travel in time by transfering memories (and/or state of mind) works?
I think it is implied that the physical tiredness isn't transfered as Okabe seems to be able to stay awake without physically suffering from it after much more time than would be humanly possible.
However, the repetitive transfers seem to affect his mental health in a big way, as suggested in one of the game's endings (Suzuha's ending):
Okabe keeps on repeating the day prior to Mayuri's death because he gave up on trying to prevent her death but doesn't want her to go away. After dozens if not hundreds of "rewinds", he ends up going mental and having homicidal thoughts. "What would happen if I just give Daru a push when that truck passes by ? Would anything really change if he dies ? I would be able to leap back to the beginning of the day anyways..." (The wording might not be correct, this is just my recollection of this ending). He ends up getting help from Suzuha.