Sleep waxes and wanes through the lunar cycle

A group of sleep-scientists walk into a bar… and after a drink or two, start asking how a full moon can change your nightly snoozes *. Funny how science works sometimes.

Lucky for the scientists, the data was already available. Roughly a decade ago, the scientists recruited 33 volunteers – both male and female, and young and old – and put them in a sleep lab for a couple of nights. Back then they wanted to study how circadian rhythms changes the body’s physiological functions. The volunteers were kept in a room with constant temperature and light levels, ate regular light snacks and had absolutely no indication of time. (Sounds a little like solitary confinement to me, eek!)

While they snoozed, scientists recorded the electrical activity of their brains with electroencephalogram (EEG). During sleep, the brain does not completely shut down – instead the ebb and flow of various neurotransmitters switches the brain from awake to unconscious and back again. EEG monitors different stages of electrical activity, which occurs in ~90min cycles, and gives information on sleep structure (see graph below). Scientists also checked for eye movement, which occurs during rapid-eye movement (REM) sleep –often associated with dreaming – and the levels of two hormones, melatonin and cortisol. Melatonin helps regulate the sleep-wake cycle by inducing drowsiness (hence their reputation as an OTC sleep aid), while cortisol is associated with the stress response. Scientists also asked the volunteers to rate their sleep quality after the experiment.

Stages of sleep. Source: Nature Outlook Special Issue on Sleep.

Stages of sleep. Source: Nature Outlook Special Issue on Sleep.

Fast-forward a decade. Data in hand, scientists went ahead to see what they could find (ie they didn’t have an a priori hypothesis). As you can see below, regardless of age or gender, sleep latency correlated beautifully with the lunar cycle, with volunteers requiring roughly 5 MORE minutes to fall asleep during the full moon. On nights close to a full moon, the volunteers also experienced 20 min LESS total sleep time, WORSE sleep quality, and ~30% DECREASE in deep-sleep. The level of melatonin also tanked around full moon, while cortisol didn’t significantly change.

Figure 1 from the paper. Sleep latency = time it takes to fall asleep. Lovely correlation!

Figure 1 from the paper. Sleep latency = time it takes to fall asleep. Lovely correlation!

Does our sleep ebb and slow with the moon then? Since the scientists didn’t think of their hypothesis until years after the experiment, the volunteers were in essence “blinded” to the lunar phase. Even if some volunteers were keen on observing the moon, there’s arguably little possibility that they might think that a full moon would affect their sleep negatively and generate placebo effects. The volunteers were kept in a room with constant levels of ambient light – hence the difference in sleep cannot be explained by different levels of light from the moon. It’s possible that ambient evening light might “prime” the volunteers to subconsciously alter their sleeping patterns before they enter the lab – but there’s no evidence for this.

One major problem with retrospective studies like this one is that it can sometimes lead to false positive statistical correlations. 33 people is a small sample, especially considering their diversity in terms of age and sex, it’s highly possible that the results would not stand up in a much larger study. (There are also people conducting n=1 “quantified self” experiments across the internet, and while interesting, can’t be counted as stringent scientific studies.)  It would be really interesting to follow up this study with a larger sample designed to directly test this hypothesis, and see if the same phenomenon can be repeatedly detected in different cohorts.

Until the results are repeated and the mechanisms unravelled, I’m still skeptical. But let’s say that human sleep does undulate with lunar phases. How could this work? Although oceanic tidal waves undulate with different lunar phases, the human body (or the water within) does not, so changes in water flow in the body is out of the question. Another long-standing myth is that lunar phases affect people’s mood and behaviour (hence words like “lunacy”), and mood may in turn effect sleep. However, while legends and old wives tales persist, this myth has been rebuked by science.

The authors suggest that perhaps just like daily circadian rhythms, humans also have a “circalunar rhythm”, which has been characterized in certain marine animals like the Galapagos marine iguanas. However, perhaps due to huge individual differences in sensitivity to lunar phases, the idea of a circalunar clock is still controversial.

The idea of a mysterious lunar clock in humans is tantalizing. What does it DO? How much influence does it have on us? How much is it disrupted by our current society of artificial lights and hectic work schedules? What are the neuronal underpinnings of such a phenomenon? Would chronic disruption of the lunar clock lead to various diseases, or vice versa?

* Quote from the methods section of the paper: "Thus, the aim of exploring the influence of different lunar phases on sleep regulation was never a priori hypothesized, nor was it mentioned to the participants, technicians, and other people involved in the study. We just thought of it after a drink in a local bar one evening at full moon, years after the study was completed. " BOSS!

Christian Cajochen (2013). Evidence that the Lunar Cycle Influences Human Sleep Current Biology DOI: 10.1016/j.cub.2013.06.029