A study in the US has found that sleep cycles are now more likely to revolve around TV viewing times than cicadian rhythms. The US Department of Labor Statistics found that most people went to sleep after switching off the TV around 11-11.15pm at night - while the time that nightfall actually occurred made very little difference to the time people slept:
While natural daylight patterns have some effect on people's life patterns, the demands of global business—market openings, etc—and regular television schedule demarcate the boundaries of most Americans' lives . . . Individuals in early television zones (Central and Mountain) are 6.4 percentage points less likely to be watching television between 11 and 11:15 p.m. than those in later zones, but if the sunset is pushed back by an hour the probability of watching TV at 11pm only increases by about one percentage point. The implications for people who want to change their sleep patterns — to get up earlier, say, or go to bed at a regular time — are enormous. If you are somebody who watches TV, you can simply turn the TV off earlier and give your body a cue that it's time to sleep.
As Analee said over at IO9, it's cool to think it can be that easy to reprogram your sleep cycle, but creepy that most Americans' sleep patterns are now controlled by the TV!