It really depends on the situation for all people. Firstly there is the sleeping pattern of the person. If they regularly sleep at a certain time, then the brain will recognise that that time is used for rest and therefore making it quicker for the person to fall asleep.
On the Extreme side is the people who sleep at random hours. For people like these its a matter of sleeping when the brain gets tired. Thats why people can take up to 3 hours to fall asleep, the brain doesn't seem to recognise that its the time for rest and therefore continue to function until you tire out. Explains why people tend to have so many things on their minds if they can't fall asleep.
