Waking gradually with light seems like it should make mornings easier. The evidence is more precise about what that light can actually do.

A sunrise alarm clock makes an appealing promise: that waking does not have to begin with the sound of an alarm.

The clock uses light to simulate a sunrise in your bedroom. The light starts dim and gradually gets brighter, creating an artificial dawn while you are still asleep. The idea is that this gradual increase in light makes the transition from sleep to waking feel easier and less abrupt.

But does it actually work? Does the light make waking easier? And because light can influence the body clock, does a sunrise alarm change your circadian timing too?

The clearest evidence is about waking

The strongest findings are about the moment of waking itself. Most of us know the feeling of grogginess that lingers after the alarm goes off, when you are awake but still feel slow to get going. Researchers call this sleep inertia: the period of reduced alertness and impaired performance immediately after waking.

A 2010 study led by Van de Werken found that dawn simulation, light that increased gradually before waking, reduced how sleepy people felt on waking and eased the transition out of sleep. Another study that year, led by Giménez, found that artificial dawn reduced people’s subjective complaints of sleep inertia.

So there is evidence that gradually increasing light before waking can make the transition out of sleep feel easier. The findings are modest, and they are based largely on people’s subjective experience of waking. Exactly why the light helps is less clear. Cortisol naturally rises around waking and has been suggested as one possible explanation. But in the Van de Werken study, people reported waking more easily without a significant change in the cortisol response.

Feeling more awake is not the same as moving your body clock

This is where the more interesting question sits. If people wake more easily with a sunrise alarm, has the light shifted their body clock, or has it only changed how waking feels?

The Giménez study is useful precisely because it looked at both. Alongside the reports of reduced sleep inertia, the researchers measured dim-light melatonin onset, or DLMO: the point in the evening when melatonin begins to rise under dim light, which researchers use as a marker of circadian timing. After two weeks of artificial dawn, that marker had not significantly shifted.

This does not mean light at dawn cannot affect circadian timing. In 2000, Danilenko and colleagues showed that a dawn signal can affect circadian timing under controlled conditions. But that is not the same as showing that an ordinary sunrise alarm on a bedside table reliably shifts your body clock. The evidence that these devices make waking feel easier is stronger than the evidence that they reliably move your circadian timing.

What about feeling more alert once you are up?

A small 2014 study led by Thompson found improvements in some measures of alertness and performance after waking with dawn simulation. In 2013, Gabel and colleagues found benefits for daytime cognition and well-being, although the participants were sleep restricted at the time.

These studies suggest there may be benefits beyond the initial transition out of sleep. But there is not enough evidence to say that a sunrise alarm will leave you more alert, improve your performance or lift your mood after you are awake.

Is a sunrise alarm worth trying?

Someone who dislikes being jolted awake in a dark room, or who struggles with grogginess in the first part of the morning, has good reason to try one. Someone hoping it will turn them into a morning person, or move the timing of their body clock, is expecting something the evidence does not support.

For some people, making waking a little easier may be reason enough.