Although it's far from settled, there is quite compelling evidence that the advent of the widespread electric light could have disrupted our natural sleep patterns to the point where we now need alarms to raise us from slumber.

A. Roger Ekirch - history professor at Virginia tech - has put forward the theory in his book 'At Day's Close: Night in Times Past' and several articles, most notably his article for the American Historical Review 'Sleep We Have Lost: Pre-industrial slumber in the British Isles' that humans natural sleep pattern was biphasic - going to sleep after sunset, waking for an hour or two in the night, then returning to sleep until sunrise.

Ekirch gives a number of examples from the middle ages through to the industrial revolution where people write of 'first sleep' and 'second sleep', probably the most famous of which is Chaucer's casual mention of it in 'The Canterbury Tales'.

I am unaware if any recent scientific research has strengthened or weakened Ekrich's hypothesis.

Before alarm clocks were available or reliable, we have to remember that societies were far less neurotic about timekeeping than we are today. People in towns tended to rise with the church bells or similar local calls; in British factory and mining towns a Knocker upper was sometimes employed by the companies to go house to house and awaken shift employees. Away from the towns, people tended to rise with the sun, or with the dawn crow of the rooster.