Obviously one of the major factors that can effect getting to sleep and staying asleep is light. Light effects our sleep in two ways. The way that most of us already know is the direct effect that light has, in that if it is too light we will have trouble falling asleep and if someone turns the light on while we are asleep we will often wake up.
However, light also affects our sleep indirectly. It does this by influencing our internal body clock and thus affecting when we feel tired. We have specialised light sensitive cells in our retinas, cells which are found between the rods and cones that allow us to see. These cells tell the brain when it is day and night and the brain uses this information to set our sleep patterns to match the circadian rhythm (the cycle of day and night we experience).
The invention of the light bulb in the late 19th century has meant that we are exposed to far more light during the day than we use to be and it has had an effect on our sleeping patterns. This exposure to light in the late evening tends to delay the phase of our internal clock, meaning we feel tired later and thus get up later.