As described by Stephanie Hegarty for the BBC, Roger Ekirch, an historian, has evidence — and is convinced — that people used to have
a first sleep which began about two hours after dusk, followed by waking period of one or two hours and then a second sleep…. During this waking period people were quite active. They often got up, went to the toilet or smoked tobacco and some even visited neighbours. Most people stayed in bed, read, wrote and often prayed.… Ekirch found that references to the first and second sleep started to disappear during the late 17th Century. This started among the urban upper classes in northern Europe and over the course of the next 200 years filtered down to the rest of Western society.
Staying up all night learning Torah is, well, hard. Especially, it's not conducive to a good shacharis (for those who say shacharis right after; not all do). I'm wondering whether there's any evidence of the custom having been to learn instead of only the first sleep (but to sleep the second sleep), or to learn instead of only the second sleep (but to sleep the first). Or whether, on the contrary, the custom has very clearly always been to learn instead of both sleeps.