Does charging your phone overnight make you fat?

I doubt it.

The Independent reports that “Charging your phone in your bedroom could make you put on weight”. It goes on to state that “artificial light from phone screens, street lights, laptops or television stops the body generating a hormone that combats obesity”.  This is not a new story – similar headlines were making the rounds earlier this year (see NHS Choices for that coverage).

However, reading a bit further down the article (and a similar one in The Telegraph, it rapidly becomes clear that this is a vast overstatement of the study’s findings. In fact, light from screens (or any light manipulation at all) was not investigated in the study. Nor, indeed was weight loss. The study itself, which was pharmacological in nature, examined how the addition of melatonin (one of the hormones involved in sleep/wake cycles) in animals’ drinking water over a period of six weeks affected the functioning of a particular portion of the cell – the mitochondria – in obese, diabetic mice. The upshot was that the extra melatonin consumed was associated with improved mitochondrial function when compared to littermates who did not receive the intervention. Clearly, there have been a few huge leaps from the findings of the study to the focus of the news stories. A better headline for this study might have read: “Do melatonin pills make your cells (or those in lab animals) work better?”.

It is, however, fairly well established that endogenous (internally-generated) melatonin interacts with external light cues, with melatonin production increasing when it gets dark. Indeed, it is this ‘entrainment’ to external light cues which drives our circadian rhythms – why we get tired at night and why we wake up in the morning. And it has also been shown that, among many other things, mitochondrial dysfunction is related to obesity and type 2 diabetes. So is it really that big a leap to make that too much light at night would make you fat?

Yes. For one, the directionality of the study goes in the opposite direction – increasing melatonin affecting a variable does not necessarily make the converse true. A pharmacological reduction in melatonin from normal levels, leading to an increased mitochondrial dysfunction and subsequent obesity, would be a better match to the headline. Furthermore, the link between light levels, wavelengths of light and melatonin inhibition is far from clear in humans (although enough evidence exists of such a link for the author of this piece to install F.Lux on all of his devices…). On top of all of this, the phone-charging angle is just bizarre. Ironically, my devices shift on to sleep mode when charging. No light is emitted in that mode, so it wouldn’t be charging your phone per se that’s of interest, but any indirect exposure to light emitted from it during the night when receiving notifications/messages (if you’ve left it on), or in cases when a person is directly interacting with their phone.

So why has this study received such flaky news coverage? For one, the study itself is a culmination of an impressive and long-term programme of work, rather than a huge standalone finding. The temptation in these cases is to include a lot of background information, which gets absorbed into the findings when reported. Furthermore, the press release is quite an odd mix of an overly technical summary and more general details. Critically, it does not include the full citation (and, as a consequence, neither do the news articles, which we discuss in our How to “Research the Headlines” series; we’ve tracked down the citation in this case and as always at Research the Headlines, have included it below). I suspect that the news articles have latched upon a brief quote toward the end of the press release referring to  many peoples “…habit of sleeping with their lamps, TVs or their computers switched on…” in lieu of getting a clear message from the press release itself.

Jimenéz-Aranda, A., Fernández-Vázquez, G., Mohammad A-Serrano, M., Reiter, R. J., & Agil, A. (2014). Melatonin improves mitochondrial function in inguinal white adipose tissue of Zücker diabetic fatty rats. Journal of Pineal Research. DOI: 110.1111/jpi.12147