You love to say you're a night owl, yet your body is telling a very different story. Your circadian rhythms, the routine changes in your behavioral, mental and physical functions that occur over the course of a day, are regulated by a tiny area of the brain commonly known as your 'biological clock.' The suprachiasmatic nucleus (SCN) is shaped like a pine cone, but is about the size of a grain of rice, though it contains about 20,000 neurons.

When light enters your eye, it activates neurons in the retina that convert photons (light particles) to electrical signals. These signals travel along the optic nerve to the SCN which in turn stimulates several brain regions, including the pineal gland. The pineal gland responds by switching off production of the hormone melatonin, and this makes you feel more awake. The SCN also governs your body temperature, hormone secretion, urine production, and changes in blood pressure. After darkness falls, the SCN signals once again and your body's level of melatonin increases, making you feel drowsy.

This intricate and complex biological system responds daily and automatically to the rhythms of day and night. Like all biologic beings, you respond to your environment. You are a part of the whole. When you impose an artificial tempo to your day by going to bed too late, there will be real health consequences.

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.


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.


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.

Cardiovascular risks

Reduced sleep is also related to an increase in both cardiovascular events and cardiovascular disease. In the Nurses' Health Study, researchers found evidence of increased risk of coronary events in female subjects who obtained seven hours or less of sleep per night compared to those who averaged eight hours per night. In another study, a two- or three-fold increase in risk of cardiovascular events, such as stroke or myocardial infarction, was found in subjects with an average sleep duration of five hours or less per night (or chronically having five hours or less of sleep per night at least twice per week) was reported. Similar findings have been observed in studies examining cardiovascular health in shift workers, who typically experience disruptions in their circadian rhythm. Although certain of a link between short sleep and cardiovascular risk, researchers do not understand the reason for this.


In a prospective study of 23,995 Japanese women, shorter sleep duration, specifically six hours or less per night, was associated with a higher risk of breast cancer when compared with women who slept seven hours per night. Researchers suggested that melatonin, which is secreted mainly from the pineal gland, is the key factor in this link between sleep and cancer. Shorter sleep results in a shorter duration of nocturnal melatonin secretion and lower melatonin levels have been associated with an increased risk of breast cancer.


Increasingly, scientists hypothesize that sleep deficits cause obesity. According to the authors of a study of 1,024 volunteers from the Wisconsin Sleep Cohort Study — a population-based longitudinal study of sleep disorders — too little sleep apparently alters your body's regulation of appetite-regulating hormones. Participants underwent nocturnal polysomnography and reported on their sleep habits through questionnaires and sleep diaries. In addition, each morning they gave blood samples, which were evaluated for serum leptin, ghrelin, adiponectin, insulin, glucose, and lipid profile.

After sorting through these measures as well as sleep habits and body mass index (BMI) for each of the participants, researchers identified a relationship between sleep duration and BMI. Increased BMI was proportional to decreased sleep duration for those participants who slept eight hours or less (74.4 percent of the sample) each night. Independent of BMI, short sleep patterns were also associated with low leptin and high ghrelin. Since reduced leptin and elevated ghrelin increase appetite, this likely explains the increase in BMI.

Sources: Basner M, Dinges DF. Dubious Bargain: Trading Sleep for Leno and Letterman. Sleep. 2009.

Kakizaki M, Kuriyama S, Tsuji I. Sleep duration and the risk of breast cancer: the Ohsaki Cohort Study. British Journal of Cancer. 2008.