PITY the poor pineal gland, tucked behind the thalamus in a gap between the brain’s hemispheres. It has a simple task—to make melatonin, a hormone that regulates sleep. In days gone by, it would start doing so after sunset, ramp up to a maximum in the middle of the night, and then taper off toward the morning. The result was regular, dependable periods of sleep and wakefulness.

Modern life, though, is confusing for the pineal because its signal to start work is the absence of light—specifically, of blue light. This part of the spectrum radiates by the bucketful from light-emitting diodes in the screens of phones, tablets and laptop computers. As far as the gland is concerned, that turns night into day. Study after study has suggested night-time use of screen-based gadgets has a bad effect on peoples’ sleep. Indeed, things are getting worse as screens get smaller and are thus held closer to the eyes. As a consequence there is a tidy market in devices and apps which regulate the amount of blue light a screen emits.

The latest research suggests one group of people—teenagers—may be particularly susceptible. Those in their mid-teens already have unusual sleep patterns. Left to themselves, they stay up late and sleep in in the morning because their melatonin cycles start and finish later than those of adults. Add teenagers’ reputations for being glued to their screens and it certainly seems reasonable to hypothesise that adolescents, in particular, will suffer from sleep-disruption-by-gizmo.

One study, published in October by researchers in Switzerland, tracked the self-reported sleepiness and alertness of boys aged 15 to 17 over the course of two weeks at home in which they wore either glasses fitted with filters that blocked blue light, or else clear glasses of similar design, for several hours before they went to bed. The team then repeated the experiment in a laboratory, and measured the youths’ melatonin levels and reaction times over the course of the evening. All the results pointed the same way: minus blue light, participants were more ready for bed.