In nature, humans are diurnal, so sleeping at night is our natural biological rhythm. In our modern world of artificial light and 24/7 activity, though, night shifts are common. Unfortunately, working the night shift can interfere with our sleep-wake cycle, and that can lead to health problems ranging from obesity to cancer to injuries. Your mental health can suffer as well.


One of the most serious health effects of night shift work is an increased risk of cancer. The effect of night shift work is significant enough that, in 2007, the World Health Organization listed “shiftwork that causes circadian disruption” as a probable carcinogen. Women who work at night, mainly flight attendants and nurses, have an increased risk of breast cancer. This increase in risk is consistent with experimental animal research showing increased tumors in animals kept in constant light, dim light at night, or simulated chronic jet lag. A number of epidemiological studies have also associated circadian disruption with cancers in diverse human organ systems, including ovarian, lung, pancreatic, prostate, colorectal, and endometrial cancers, non-Hodgkin’s lymphoma, osteosarcoma, acute myeloid leukemia, head and neck squamous cell carcinoma, and hepatocellular carcinoma.

 Other Chronic Health Problems

In addition to cancer, epidemiological studies have found an increase in cardiovascular disease in night shift workers. Shift work is also associated with an increased rate of gastrointestinal, metabolic, and reproductive problems. Moreover, night shift workers show greater rates of overweight and obesity. According to the Centers for Disease Control and Prevention, night work is also associated with an increased risk of diabetes, impaired immune function, and aging.

 Mental health and well-being

Working the night shift can lead to impaired mental and physical functioning, including emotional fatigue and a weakened immune system. The sleep disorder known as shift work disorder leads to a number of psychological difficulties, including difficulty concentrating, insomnia, sleep that feels unrefreshing or insufficient, and excessive sleepiness when you need to be awake. Night shift work also tends to strain personal relationships, such as marriage and family. As a result, night shift workers suffer from a higher risk of depression.


When we don’t get enough quality sleep, we wake up less alert and able to concentrate, make decisions, and engage fully in activities.  This can lead to injuries from accidents, from car crashes to needle sticks.  Especially at risk are transportation workers, such as truck drivers who drive long distances at night, an especially high-risk profession.  Nurses working at night, another occupation with substantial risk, are injured more often than those working during the day.  According to a recent review by University of Basel researchers Uehli and colleagues, approximately 13% of work injuries could be attributed to sleep problems.

 Solutions: sleep hygiene, light therapy, pharmacology, and daily exercise

As living beings, we have a hard-wired biological clock known as the circadian timing system. This circadian rhythm is based on two things: an endogenous timing system of approximately 24 hours, and a system that takes into account cues like daylight and activity. In order to minimize problems associated with night shift work, you can do two things. One is to try to make the night shift more regular, in line with our natural 24-hour biological clock. If you rotate, it is better if you work the night shift for a few weeks, then do the day shift for a few weeks, rather than doing a few night shifts and a few day shifts every week. In addition, try to shift in the positive direction—that is, change your shifts so that you are waking up later for the new shift rather than earlier.

The other thing you can do is adjust your environmental cues. Light is a big one. Make sure that your sleeping area is darkened when it’s time to sleep, and if you wake up in the dark, use a light therapy box. Light therapy boxes emit either bright white or blue light. Simply sit in front of the box, with the box shining on your face at about a 45 degree angle, for about 15 minutes when you first wake up. You will find that you are less sleepy throughout the day, and that you will adjust more quickly to the new routine; people often wake up just before their regular light box time, just as they would wake up with the sun on a natural schedule. Exercise is also key; regular exercise during your awake time can counteract many of the health effects associated with night shift work. Some individuals find the natural supplement melatonin, taken just before the time they want to go to sleep, to be helpful. Finally, while medications can help you get to sleep and stay awake, these are options of last resort, as they can be habit-forming and can be harmful to natural sleep architecture. Your doctor can help you decide whether it makes sense for you to take any medication, and if so, which type would be best for you.

References and further reading:

Journal of Advanced Nursing: Twenty-Four/Seven: A Mixed-Method Systematic Review of the Offshift Literature

International Agency for Research on Cancer, World Health Organization

Progress in Molecular Biology and Translational Science: The Circadian Clock in Cancer Development and Therapy

Progress in Brain Research: The Impact of the Circadian Timing System on Cardiovascular and Metabolic Function

Sleep Medicine Reviews: Sleep Problems and Work Injuries: a Systematic Review and Meta-Analysis

Journal of Nature and Science of Sleep: Shift Work: Health, Performance and Safety Problems, Traditional Countermeasures, and Innovative Management Strategies to Reduce Circadian Misalignment

National Sleep Foundation: Shift Work and Sleep

Centers for Disease Control and Prevention (CDC): Sleep and Work

National Sleep Foundation: Shift Work Disorder

Healthcare Journal of New Orleans: The Therapeutic Benefits and Risks of Light

Centers for Disease Control and Prevention (CDC): Truck Driver Safety and Health

CDC Pamphlet: Quick Sleep Tips for Truck Drivers

National Sleep Foundation: Non-Medical Treatments for Shift Work Disorder


So-called “good parasites” in the human body have been linked to protection against a number of disease conditions, from colon cancer to obesity. They also perform beneficial functions like synthesizing vitamins and helping to protect against infection from pathogenic microorganisms. These organisms living inside us are not, by definition, parasites. Symbionts, or organisms that live inside us or on us, can be divided into three types: parasites, which cause us harm, mutualists, which help us or do us good, and commensals, which do not necessarily do us good, but which do not hurt us. The “good parasites” are the mutualists.

One of the most important categories of mutualists is that of the intestinal bacteria. The colon is colonized by more than a trillion organisms per gram of intestinal contents. In fact, there are more bacterial cells than human cells in our bodies! These include the familiar E. coli, a natural gut-inhabiting bacterium, as well as bacteria commonly ingested in probiotic foods, such as Lactobacillus from yogurt. The vast majority of the bacteria fall within four phyla (major groups): Firmicutes (including Lactobacillus), Bacteroidetes, Actinobacteria, and Proteobacteria (including the familiar E. coli). Methanobrevibacter smithii, an archaean (a microorganism that looks like a bacterium but is not a true bacterium), is another major intestinal resident in humans. Minority gut residents include the fusobacteria, cyanobacteria, spirochaetes, and verrucomicrobia. Probiotic supplements can help you boost many of these good symbionts.

In addition to these directly beneficial bacteria, there may also, paradoxically, be benefits to infection with true parasites that cause disease. Illustrating the axiom “what does not kill me makes me stronger”, immunologists have found that infection with parasites can aid in the healthy development of the immune system. The prevention of infection with helminth parasites (worms) and other infectious organisms in developed countries has been linked to increased rates of allergy and asthma, and this is thought to lead to abnormal development of the immune system. It seems that such infections are integral to the natural “wiring” of the immune system. Without them, immune systems can develop inappropriately, attacking benign substances such as pollen—these are allergic reactions. Infectious organisms that have been shown to provide protection against allergy include the nematode (roundworm) parasites Heligmosomoides polygyrus, Nippostrongylus brasiliensis, and species of Ascaris; the platyhelminth (flatworm) parasites Schistosoma mansoni and Schistosoma japonicum, bacteria such as Mycobacterium, Chlamydia, and Listeria, viruses, including influenza virus and respiratory syncytial virus, and even fungal pathogens, such as Aspergillus fumigatus.

Clearly, the microbiome, the population of organisms that live inside us, is an important part of who we are, playing roles in gastrointestinal function, immunity, and other aspects of human health. Future molecular and genomic studies should lead to further discovery of as-yet unknown mutualistic organisms.

References and further reading:

Tsai F, Coyle WJ. (2009)The microbiome and obesity: is obesity linked to our gut flora? Current Gastroenterology Reports: 11(4):307-13.

Kinross JM, von Roon AC, Holmes E, Darzi A, Nicholson JK. (2008) The human gut microbiome: implications for future health care. Current Gastroenterology Reports: 10(4):396-403.

Bik EM. (2009) Composition and function of the human-associated microbiota. Nutrition Reviews: 67:S164–S171.

Ley RE, Peterson DA, Gordon JI. (2006) Ecological and Evolutionary Forces Shaping Microbial Diversity in the Human Intestine. Cell: 124(4):837-848.

Roumier T, Capron M, Dombrowicz D, Faveeuw C. (2008) Pathogen induced regulatory cell populations preventing allergy through the Th1/Th2 paradigm point of view. Immunologic Research: 40(1):1-17.

Lice, Mites, and Worms, Oh My!  Metazoan Parasites of Children in New Orleans