DSPS is believed to be a disorder of the body's timing system - the biological clock. DSP patients have fifficulty falling asleep and difficulty waking because their biological clocks are out of phase with the sleeping and waking times they try to carry out. DSPS is similar to jet lag, but much longer-lasting. It can develop suddenly or gradually.
You probably have heard of a biological clock which governs growth, reproductive cycles, and aging. There are also bodily rhythms, known as circadian rhythms, which are also controlled by a biological clock and which work on a daily time scale. You might have already noticed, in yourself or in others, that sleepiness doesn't just keep increasing as it gets later. Rather, the drive for sleep follows a cycle, and the body is ready for sleep and for wakefulness at different times of the day.