Shorter, more concentrated study sessions distributed over a period of days or weeks will be much more effective in the long-term than massive day-long cram sessions. Mental exhaustion is a very real phenomenon, which is why it's good to mix up the types of subjects you're studying during any given session to avoid burning out from studying one particular topic for too long a period. Also, don't pull all-nighters if you can avoid it.
Sleep is vital to strengthening mental connections made throughout the day and flushing built-up neurotoxins from the brain, and sleep deprivation can cause demonstrable harm to your ability to think clearly the next day.
