Animal studies replicate these findings. According to a 2014 study published in the journal Cancer Research, animal researchers linked the fragmented sleep from sleep apnea with accelerated cancer growth in mice. The researchers placed mice with tumors in low-oxygen environments (to mimic the effects of having sleep apnea), and their cancer progressed at an accelerated rate.
Researchers theorize that the oxygen deprivation associated with sleep apnea may be what’s causing the cancer. When the body doesn’t get enough oxygen, it grows more blood vessels as a way to compensate, which in turn enables cancer tumors and tissue to grow faster.
Shift work sleep disorder and cancer
Researchers have established a link between circadian rhythm disorders, in particular shift work sleep disorder, and increased risk of breast, colon, ovary, and prostate cancers.
Circadian rhythm disorders occur when an individual’s biological clock becomes out of synch with the external environment. Many of us have experienced jet lag, a circadian rhythm disorder that happens when you travel across time zones and your body hasn’t quite synchronized with the day-night cycle of your new location. Shift work sleep disorder affects shift workers, especially those on night shifts or rotating shifts, who are awake during times when the body expects you to be asleep. Common symptoms include insomnia, microsleep episodes, fatigue, and poorer mood and concentration.
Multiple studies have studied the effects of shift work on breast cancer risk. Women who worked night shifts for 4 years and those who worked fewer than 3 night shifts per week both had a 30 percent higher risk of breast cancer than women who did not perform shift work at all, according to a 2012 study published in the International Journal of Cancer. In the first group, the women had entrained their bodies to the opposite of the normal day-night cycle, having worked night shifts regularly for several years. The second group of women never got a chance to fully get used to one schedule over the other, daytime or night. Both scenarios increased their risk of breast cancer.
Even if shift workers get sufficient sleep, researchers suspect the issue may be the disruption to their “normal” circadian rhythms. During shift work, the body is exposed to light during times when it should be asleep. Besides regulating your sleep and wake cycle, your circadian clock directs a variety of biological functions, including organ function, hormone production, and body temperature. The impact to these other functions could also create a conducive environment for cancer to grow.
Most critically, though, disrupting your circadian clock can delay melatonin production (the hormone responsible for sleep) and impact the body’s overall levels of melatonin. When scientists alter the sleep wake cycles of rodents in the lab, cancer also grows at an increased rate. Researchers suspect these decreased melatonin levels may facilitate cancer growth.
For example, a 2003 Boston study found reduced melatonin levels were associated with an increased risk of breast cancer. Here, the researchers think the issue lies with the increased estrogen production, which occurs when melatonin levels are lower. Increased estrogen production is yet another risk factor for breast cancer.
Further cementing the link between melatonin levels and cancer, a Michigan State University study found that treating breast cancer stem cells with melatonin reduced the number and size of breast cancer tumors, suggesting that the antioxidant properties of melatonin could eventually be used as a treatment for stopping the growth of breast cancer tumors.
The increased cancer risk for shift workers may also have something to do with their cortisol levels. Cortisol production operates in inverse to melatonin – cortisol levels peak in the morning after sleep and decline during the day. Female night shift workers have a “shifted cortisol rhythm” and peak in the afternoon instead of at dawn when they’re supposed to.
How does cancer affect sleep?
Between 30 to 75 percent of people receiving cancer treatment have some sleep problems and about 25 percent of cancer survivors continue to have issues sleeping. Lung cancer patients tend to report the most sleep problems.
The side effects of cancer and treatment cause a host of issues that can make falling and staying asleep more difficult. Individuals with cancer may have increased anxiety and depression, two conditions that go hand-in-hand with insomnia. Extensive treatment can cause excessive fatigue and cancer-related sleep disorders. Hot flashes and night sweats are common side effects that make it challenging to get comfortable enough to fall asleep.
Generally, the three largest sleep issues affecting cancer patients are insomnia, excessive daytime sleepiness, and restless legs syndrome.
While insomnia isn’t typically a common side effect of the chemotherapy itself, many of the associated medication, especially steroids, can cause disrupted sleep. Worse, insomnia often exacerbates other symptoms and side effects from the cancer treatment. Some estimates report insomnia affecting one-third to one-half of cancer patients, which is two to three times higher than the general population.
Insomnia describes a general difficulty falling or staying asleep on a regular basis. Among cancer patients, the most commonly reported symptoms of insomnia include:
- Frequent nighttime awakenings (76%)
- Difficulty falling asleep (44%)
- Waking up too early (33%)
Chemotherapy drugs can cause fatigue, so patients are more prone to nap during the day, leading to insomnia at night. The drugs used to manage the side effects of chemotherapy can also cause sleep problems: anti-nausea medications may cause drowsiness, while the energizing nature of steroids make it difficult to fall asleep.
Combine all this with the emotional distress of having cancer, and the anxiety and worry which may keep one up at night.
Excessive daytime sleepiness and fatigue
Excessive daytime sleepiness and fatigue are highly correlated with insomnia. In one study of cancer patients, those who reported fatigue were 2.5 times more likely to also have insomnia.
While fatigue and excessive daytime sleepiness are clinically distinct, they can feel similar to the individual living with them. Fatigue refers to a feeling of low energy and exhaustion, which often causes the individual to seek napping as a way of relief. As alluded to above, fatigue is a common side effect of cancer treatment. Individuals undergoing radiotherapy reported nearly double the amount of fatigue after receiving radiotherapy than before treatment. Fortunately, the fatigue returned to normal levels after a period of months.