But then some smart guy went and developed electricity, and another genius made light bulbs, and things have pretty much been downhill since then. Scientists at the University of Washington decided to study how access to electricity affects people's sleep, and they came to the groundbreaking conclusion that artificial light sources led humans to sleep 1) later and 2) less.
The trouble is, we really can't just sleep "whenever" and expect to get adequate rest. Our bodies are hardwired to go to bed around sundown, and the quality of our sleep diminishes as the night wears on -- moving from a more restorative deep sleep, toward a lighter REM sleep that includes dreaming. If you go to bed relatively early -- say 9 pm -- your body should get multiple cycles of deep sleep before getting to the dream-heavy hours of the early morning. But if the early morning is your bedtime, your body can't just start with the deep sleep cycles that are typical of earlier sleep. So basically you get all the weird dreams about your teeth turning to sand, TV characters mocking your genitals, or unicorn proms, but none of the stuff that actually helps repair your body.
Studies have shown that the genes of people working the night shift never adapt to the body's immune or metabolic processes to accommodate a reversed schedule. Our bodies have a circadian rhythm reinforced by hundreds of thousands of years of biology, and that's almost impossible to override. Need another reason to hit the sack early? Research shows that the later you go to bed, the more likely you are to lay awake obsessing over negative thoughts, and the later your average bedtime, the less you will probably sleep overall.