Muhammed bin Abi Haatim (rahimahullah) narrates the following:

When I would accompany Imaam Bukhaari (rahimahullah) on a journey, then we would stay in the same room, unless it was the summer season (when the heat would be intense).

During the night, I would notice that Imaam Bukhaari (rahimahullah) would get up approximately fifteen to twenty times. On getting up, Imaam Bukhaari (rahimahullah) would take the flint (a stone used for lighting fire) and use it to light the lamp. Thereafter, he would look at certain Ahaadith and make notes. After some time, he would place his head down and go back to sleep.