Is it safe to wear a tampon overnight? i've heard that you shouldn't. is it because of the length of time perhaps?

Likely okay: The concern is toxic shock syndrome. A very rare occurrence that can happen if you leave a tampon in too long and it causes an infection. Surveys of women show many use tampons overnight and don't get toxic shock, so you're likely okay. If you're worried though, can consider using a pad at night. All gynecologists have had patients where tampon left for days (patient often forgets to remove).