Type of Sleep
Sleep is a cycle of phases, a shift back and forth between rapid eye movement (REM) sleep and non-REM sleep. During REM sleep, memories and thoughts from the day are processed; it is the stage of sleep in which vivid dreams occur. During non-REM sleep, many restorative functions occur. In the deepest phase of non-REM sleep (known as slow-wave sleep), the brain recovers from its daily activities and hormones are released, which help the body rebuild itself from damage done during the day.
After you fall asleep, you cycle through 90-minutes of non-REM sleep followed by REM sleep.
Oddly, the ratio of non-REM-to-REM sleep within these 90-minute cycles changes across the night, regardless of when you go to bed. Early in the night (between the hours of 11 p.m. and 3 a.m.), the majority of those cycles are comprised of deep non-REM sleep and very little REM sleep. Yet, in the second half of the night (the hours between 3 a.m. and 7 a.m.), this balance changes; the 90-minute cycles are comprised of more REM sleep together with a lighter form of non-REM sleep. Because there exists a greater propensity for deep non-REM sleep earlier in the night, someone who sleeps from 9 p.m. to 5 a.m. (eight hours total) will have a different overall composition of sleep with more non-REM than someone who sleeps from 3 a.m. to 11 a.m. (also eight hours total) and so is likely to experience more REM.
Going to bed too late, then, will deprive you of some of the restorative functions that non-REM sleep normally provides.
Insomnia
A nearly immediate effect of going to bed after midnight is that it throws off your natural circadian rhythms, governed by your SCN, and this may lead to insomnia. As well as having a harder time falling asleep, you will also have trouble staying asleep. Even if you have a schedule that allows you to wake later, the rest of the world will not readily accommodate you. Often, the noise and commotion of the day will in all likelihood wake you before you wish; it is a well-documented fact that night shift workers, despite having a schedule that allows for an adequate amount of sleep, get less sleep than those who work days.
Along with anxiety, a whole host of negative consequences flow from this basic fact of sleep deprivation