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.
Accidents
In the workplace, the effects of too little sleep are revealed by reduced efficiency and productivity, errors, and accidents. Yet decreased driving ability is a very real-world risk associated with sleep debt. One study found an increased incidence of sleep-related crashes in drivers reporting less than seven hours of sleep per night on average. Additional contributing factors to these crashes included poor sleep quality, dissatisfaction with sleep duration, and daytime sleepiness. According to the National Highway Traffic Safety Administration, "drowsy driving" causes over 100,000 car crashes each year, resulting in about 1,550 deaths.
Immune response
The impact of too little sleep on the immune system has not received very much attention even though sleep deprivation has been shown to activate defense mechanisms and to elevate certain inflammatory cytokines or cell "messengers" (IL-6, TNF) in young adults. In an experiment, healthy participants had their sleep restricted to six hours per night; results found the 24-hour secretory profile of IL-6 was increased in both sexes and TNF-alpha was increased in men. Both IL-6 and TNF-alpha are markers of systemic inflammation, which causes pain and soreness and may lead to osteoporosis or autoimmune diseases.
In another study, researchers reported sleep restriction decreased antibody production to vaccination. In participants vaccinated for influenza immediately following a six night run of short sleep (four hours per night), the level of antibodies decreased by more than 50 percent 10 days following the vaccination compared to those vaccinated after a six night run of regular sleep.