When you close your eyes and get ready to drift off, our minds tend to wander. Sometimes we play back an embarrassing part of our day, something we have to do tomorrow, or we just think about how we’ve definitely over-eaten and are now struggling to fall asleep. Earlier this week, as I was struggling to drift off, I was thinking about how sleep really works. I mean, how does lying down with our eyes closed actually help us rest? And then, as the diligent researcher I am, I thought, “I wonder if sleep is at all related to cancer.” Let’s try to answer those questions.

You may have heard that when we sleep, we go through roughly 90-minute cycles. That’s because there are different phases of sleep, each with a different purpose. We start in something called “non-rapid eye movement 1”, which is also called NREM 1. This is light sleep, the transient period into deeper sleep. Next comes NREM 2, a deeper stage of sleep than NREM 1, and it serves several important purposes. NREM 2 is essential for building memories. That includes both general memories and muscle memory.

Next, we have NREM 3. At this point, our sleep helps with more physical aspects. We repair tissue, build muscle and bone, and strengthen our immune system. This is more of the physical resting phase.

Lastly, we have an altogether different phase: rapid eye movement, or REM. REM is when we dream, and it’s absolutely crucial for emotional processing. It’s defined by rapid eye movement, and at this point, our brains are actually wide awake. If you measure the activity of your brain in REM sleep compared to when you’re awake, it’s very similar. We’re so “awake” that our bodies have to actively prevent our muscles from contracting, just so we don’t move during our sleep while we dream.

This entire cycle takes about 90-110 minutes, and we go through four or five of them for a full sleep.

This seems pretty straightforward, but it’s actually quite complex. Presently, we know that it’s essential for our health, but we almost certainly don’t have the full answer. What we do know is that each stage of our sleep plays a role in maintaining our health, both emotionally and physically. Our metabolism, respiratory system, circulatory system, and countless other processes that keep us functioning properly rely on good sleep. One factor that our sleep impacts is our balance of neurotransmitters. Neurotransmitters are molecules that our brain uses to send messages. There are many types of neurotransmitters; each neurotransmitter has a different effect. You may have heard of “the happy hormones”, like dopamine or serotonin. These are examples of neurotransmitters that regulate our mood, often making us feel “good”.

Apart from just impacting our mood, sleep deprivation has been linked to higher rates of depression and anxiety and contributes to our risk of Alzheimer's.

Sleep deprivation also has an enormous impact on our physical health. It increases our risk of cardiovascular diseases like stroke and hypertension. It increases our risk of metabolic disease, such as diabetes and the risk of obesity. Sleep deprivation also increases the risk of a variety of different factors, like inflammation, insulin insensitivity, several different causes of mortality, and the risk of cancer.

Several studies have shown that insufficient sleep has been linked with higher rates of various cancers, including prostate, breast, and colorectal cancer. The difficult part here is the “why”. There are very few studies that look specifically at the role of sleep deprivation and the biology of cancer. With that said, there are some clear possibilities.

The first is a hormonal imbalance. Several types of cancer depend on hormone availability to grow. Prostate cancer, breast cancer, ovarian cancer, and others are predominantly hormone-dependent. That means they either rely on testosterone or estrogen to grow. In healthy tissues, these hormones play a role in various functions, regulating bone density, muscle mass, and metabolism. At a cellular level, estrogen and testosterone can cause cells to divide. This would be an ideal scenario for a cancer, as they attempt to divide and grow as quickly as possible. This is supported by studies that have already shown that prolonged exposure to high levels of estrogen is linked with an increased risk of breast cancer.

Another possibility is inflammation. Inflammation is our body’s response to harm. That could include physical damage, like a cut. It can also include infection. Typically, inflammation is good for us; it helps repair damage and fight off infection. But too much of a good thing is a bad thing. So, in the case of inflammation, if we have prolonged inflammation, also called chronic inflammation, it can actually become harmful. Chronic inflammation can lead to rapid cell division, DNA damage, impaired DNA repair, and blood vessel formation, all of which create the perfect environment for cancer growth. We know that insufficient sleep directly increases markers of inflammation, but it can also lead to dietary hormone imbalance, insulin insensitivity, and ultimately diabetes, all of which can lead to an increased risk of cancer.

We know now that a lack of sleep can increase our risk of cancer, but it turns out cancer can also impact our sleep. Many cancer patients have reported difficulty falling asleep, staying asleep, and poor sleep quality. You might immediately think it must be because of treatment, which is certainly involved, but this has been reported before, during, and even years after treatment.

For example, cancer cases are associated with anxiety and depression, which can cause sleep disturbances. Cancer can also impact our hormonal balance, which can impact our sleep quality, leading to further neurotransmitter imbalance, and so on. Then there is, of course, pain, which is a deeply nuanced but enormously important part of a patient’s journey. We’ll save the discussion of pain for another time, but it goes without saying that pain can cause sleep disturbances. Ultimately, many aspects of cancer can disturb our sleep, which, as we know, can impact our risk of cancer.

To say “sleep is important” is an understatement. It truly is essential. Between REM and all the stages of NREM, sleep is critical for both our emotional and physical health. Sleep is necessary for processing emotions, storing memories, and restoring or repairing our bodies. It’s also necessary for neurotransmitter balance, which plays a critical role in regulating our mood and hormones. And because of all that, a good night’s sleep is essential for our metabolic, cardiovascular, neurological, and mental health. Knowing that, it comes as no surprise that sleep impacts our risk of cancer. The irony is that insufficient sleep can increase our risk of cancer, and that cancer can lead to insufficient sleep. And, although there’s still a lot to be discovered about how sleep impacts cancer biology and vice versa, we can use what we do know to help make decisions next time we’re lying in bed late at night, scrolling on our phones.

I hope you learned a little something today, but all this writing has really tuckered me out. I think I’m going to go take a nap.

See you next week!

If you missed last week’s letter, you can read “Not-So-Happy Accidents” to learn a little more about the history of chemotherapy.