As delicious as a perfectly brewed cup of coffee is, warm or cold, we generally know that we shouldn't pour a fresh cup after a certain time — it might keep you up at night, after all.
We also know that looking at our smartphones at night can ruin our sleep. But how many of us actually put our phones away, or at least use some sort of strategy to limit blue light from screens?
If we're slow to stash that shiny iPhone 6 Plus in a drawer because its sleep-disrupting effect doesn't seem as severe as straight-up caffeine, we can disabuse ourselves of that notion right now.
As researchers recently reported in a study published September 16 in the journal Science Translational Medicine, the amount of caffeine you'd get in a double espresso has less of an effect on your circadian clock or sleep schedule than bright light exposure at night.
Smartphones, tablets, and laptops all emit very bright blue light, which helps us see them in the sun's glare — but the effect this light has on sleep is reason to be concerned.
According to what the researchers found, exposure to bright light at night pushes your "sleep time" back approximately twice as long as caffeine does.
Comparing light exposure and caffeine
The researchers behind this latest finding were actually trying to quantify the exact effect that caffeine has on someone's waking and sleeping schedule, or circadian rhythm. This schedule is basically defined by the fact that our bodies — specifically, the pineal gland in our brain — start producing a hormone called melatonin when it's time for us to go to sleep, which tapers off as we wake up in the morning. We knew caffeine kept us up at night, but didn't have an exact measure of how it affected circadian rhythm.
Science Translational MedicineFor the study, the sleep researchers kept five volunteers in very controlled conditions for 49 days. Three hours before bed, they were exposed to one of four pre-sleep settings: caffeine and bright-light exposure, caffeine and dim-light exposure, a placebo and bright-light exposure, or a placebo and dim-light exposure.
Caffeine doses were determined by body mass, but were approximately equivalent to giving a 150-pound person a double espresso.
As you can see in the top half of the chart on the right (above, on mobile), caffeine alone (black bar) delayed what would be considered the person's "natural sleep time" about 40 minutes more than the placebo (white bar).
But bright light (white-striped bar) had an even greater effect, delaying sleep time by about 85 minutes longer than the placebo. Bright light and caffeine together (black-striped bar) led to about a 105-minute increase in the delay.
The researchers say that the differences between bright light alone and bright light and caffeine together aren't statistically significant. And in part given the extremely small size of the study, the much larger differences between bright light and caffeine aren't statistically significant either.
But in this small group of subjects, bright light showed a much stronger effect on melatonin production than caffeine alone.
It's hard to compare smartphones and coffee (but put your phone away before bed anyway)
The study here was a look at only five people, and results varied significantly within that group. As you can see in the second half of the above chart, some participants were much more affected by caffeine than others — while bright light had a much more consistent effect.
In a certain sense, these researchers showed something we all know. Caffeine keeps you up! This study helps us understand exactly how it affects our sleep schedule in particular, but it doesn't talk about some of the other effects of caffeine. Those effects include an increased heart rate, increased anxiety, and often a mood boost — all of which could keep you awake without changing melatonin production.
That leads to an important caveat: While bright light may have a stronger effect on your sleep schedule than caffeine, the complex effects of caffeine mean that it could still be more effective at keeping you awake. As researchers wrote in 2013, "increasing doses of caffeine administered at or near bedtime are associated with significant sleep disturbance." One study found that when people had 200mg of caffeine (about the same as in this new study) soon before bed, it took them 12-16 minutes longer to fall asleep — a slightly different measure than melatonin production.
Clarifying caffeine's exact effect on sleep is a complex process, and there's more to learn. But the new study does help us begin to directly compare its effect — which people seem to grasp quite readily — to the effect of nighttime smartphone habits, whose wakefulness-promoting effects are less widely known.
But while the effects on bright light on sleep are not universally known, they have been repeatedly studied, and results in this area are remarkably consistent.
Research from before the smartphone era shows that bright light delays melatonin production and sleep. Now that we all carry sun-mimicking devices in our pockets, more researchers are studying the question — and looking at screens like those on smartphones, laptops, and tablets specifically.
And while studies find different results for how much sleep cycles and melatonin production are pushed back or just impaired by bright light, especially blue light — which makes sense, since we all use different devices in different ways — research consistently shows a strong negative effect on melatonin and sleep, especially for teenagers.
So if you can, put the phone away. Your body and brain will be better off for it.