The Moon-Surgery Effect

I know a nurse who swears that the ER at her hospital is busier on nights of the full moon.


Last week a colleague of mine said to me, “Of course the moon affects our bodies – the moon can move the oceans. And our bodies are mostly water.” This was followed up by someone else pointing out a newly published article about the full-moon affecting heart surgery outcomes entitled “The influence of seasons and lunar cycle on hospital outcomes following ascending aortic dissection repair”. Its abstract concludes:

The full-moon cycle appeared to reduce the odds of death, and the full-moon cycle, along with being male and requiring a concomitant cardiac procedure, was associated with shorter LOS (length of stay).

I was not inclined to change my very skeptical opinion about the moon’s influence on heart surgery even after a quick read-through of this article . The authors’ final sentence in the article even admits, “…however, the small cell sizes for the levels of the lunar cycle warrant a further examination of this finding to rule out chance.”

I quipped, “Did the authors remember to rule out the potential effect of local gas prices on the heart surgery outcomes?”

Everyone around the table became silent.

Were they shocked by my flippant response, or were they reflecting on what I had said?

I said, “Look, I’ll see you next week. Let me put together some back-of-the-envelope calculations. I think you’ll be surprised. The moon’s gravitational pull on us is less than you might think – if I remember it correctly.”

Later that evening I found a contradicting study regarding the moon–surgery effect. This contradicting study stated:

…the risk of major morbidity after elective isolated CABG (Coronary Artery Bypass Graft) surgery was not associated with the time of surgery, day of the work week, month of the year, or phase of the moon in an analysis of more than 18,000 cases that had adequate power to detect even small differences.

This study found no connection between the full-moon and heart surgery outcomes and it had a considerably larger sample size, ~18000 compared to the ~200 in the other article.

Assuming a thorough search of the literature would find more studies supporting a moon-surgery effect and other studies finding no effect, it was time to consider the mechanism by which the moon could conceivably affect people like surgeons and their patients.

The reasonable sources for a moon-surgery affect are the fundamental forces of electromagnetism and gravity which could influence objects like the moon and heart surgeons, even if they were separated by thousands of kilometers.


The light (or photons) leaving the moon are not fundamentally different from the photons leaving the sun or the stars. I can’t imagine any effect they would have on heart surgery. It is worth pointing out that the intensity of light from a full moon is 500,000 times weaker than the intensity of light from a single surgical task light used in today’s operating rooms. Also, ceilings, and hospitals roofs, block moonlight from reaching the surgeons and the patients.


Gravity is the attractive force between all things that have mass. The fact that we are mostly water is irrelevant to gravity. We could just as easily have whiskey, or corn oil, flowing through our veins. If whiskey or corn oil catalyzed our biochemical reactions of life, gravity wouldn’t care. There is not a special attraction between the moon and our water molecules. The idea that that moon has to affect us because we are mostly water is not meaningful. The moon equally attracts whiskey and corn oil.

The moon does affect us

The moon certainly does affect us (gravitationally) every 24 hours whether we like it or not.  When the moon is on the far side of the earth, it  helps to pull us downwards – making us heavier. Approximately 12.5 hours later when the moon is more overhead, it pulls us upwards – making us lighter. This happens every day.


Pictured above are the gravitational forces from the earth and the moon acting on an average guy, “Herbert”.

Herbert weighs about 150 pounds (70 kg), and the changing gravitational force on him is about 5 milli-Newtons (mN). This gravitational fluctuation occurs every day of the year. He is 5 mN heavier when the moon and earth line up below him compared to his weight when the moon is overhead and he is at his lightest. It works out to be a difference of about half a gram for someone of Herbert’s mass.

Why don’t we notice the moon’s gravitational effects every day?

5 milli-Newtons is a very small weight difference.

  • It is one-quarter of the weight of a dime
  • It is the gravitational force of attraction between two loaded railroad cars standing side by side.
  • It is the difference in your weight when you are standing on the fifth floor of a building as opposed to standing on the ground floor.
  • It is equal to the weight of the air you exhale in one breath when you are quietly resting.

It is surprising that the gravitational effects of the moon can be offset by inhaling or exhaling. However, the moon’s fluctuating force on us can intensify from time to time. Twice a month, the earth, moon and the sun all line up; this is the lunar (or full-moon) cycle that people talk about. There is also the elliptical orbit of the moon to consider. These additional effects could conceivably add up to produce a gravitational fluctuation of about 10 mN on Herbert, or double the normal daily gravity change.  How significant is this weight change?

  • It is equal to one-tenth of the gravitational effect of having heart surgery performed in the CN Tower restaurant, approximately 400 m above an operating room at the Toronto General Hospital.
  • It is equal to one-hundredth of the gravitational effect of having heart surgery done in Singapore, rather than in Canada. (Since the earth is chubbier around the equator, gravity is naturally weaker in Singapore.)

The affect of the full-moon lunar cycle is easily overshadowed by Herbert’s choice of wardrobe for the day or if he chose to have a small sip of coffee. The loss of weight associated with Herbert choosing to go “pee” before heart surgery is orders of magnitude more influential than the moon’s and sun’s tidal forces acting on him.

What about gas prices?

I have not observed the moon affecting my mood, or my emotional state, every 12.5 hours as I would expect if the moon had that superpower. On the other hand, the price of gas at the pumps has directly, and measurably, affected my mood from time to time.

2 Responses to “The Moon-Surgery Effect”

  1. cristen says:

    How about the full moon affecting the sleep quality of cardiac surgeons?

  2. Shawn Brooks says:

    Hi Cristen!,

    Are you thinking about…
    Current Biology, Volume 23, Issue 15, 1485-1488, 25 July 2013
    “Evidence that the Lunar Cycle Influences Human Sleep”

    I wouldn’t waste money on funding a study to replicate this effect. I can think of much better uses of the money.
    What do you think?

    I wonder about all the times people have looked for a moon-sleep effect, found nothing, and published nothing about it.

    I suspect a “favourite team in the playoffs”-surgery effect would be more substantial for the surgeon.


  • Shawn Brooks

    Shawn has taught physics for many years and has also worked for a software company. He has presented at conferences and has conducted workshops for physics teachers. He is the former treasurer of the Ontario Association of Physics Teachers and has participated in and co-designed research projects pertaining to technology enhanced science education. He cares about science education and critical thinking.