We haven't been dependent on natural light from the Sun since the invention of the light bulb in 1879.

Nowadays, many people spend most of the day not just in artificially lit rooms but also looking at screens – phones, computers, and TVs. Recently, there have been concerns that looking at bright screens in the evening can confuse your circadian rhythm, which is the internal clock that regulates our sleep-wake cycle.

We would assume that this means using a screen before bed might make it harder to fall asleep. In fact, there are many products you can buy to filter out the blue light from your screens, which promise to improve your sleep quality.

Do these products actually work? Does screen light change our circadian rhythm, and does this make it harder to fall asleep? The story is quite complicated.

How does the circadian rhythm work?

The circadian rhythm is an innate 'body clock' present in many forms of life including plants, fungi, and animals. In humans, the body clock is found in the hypothalamus.

The hypothalamus releases a hormone called melatonin. Melatonin is often referred to as the 'sleep hormone' as its levels are high at night but drop just before we wake up in the morning. The clock has an intrinsic rhythm, but it can also be adjusted in response to light.

Professor John Axelsson, an expert in sleep research from the Karolinska Institute explains that the "master clock … has a near 24-hour intrinsic rhythm and is very sensitive to light around dusk and dawn, so to fine-tune the circadian system; which allows the system to be dynamic and adapt to the seasonal changes in duration of day and night."

Is technology changing our circadian rhythm?

Many aspects of modern technology, from the basic light bulb to the newest touchscreen phone, emit light. Professor Jamie Zeitzer from Stanford University says, "light is mainly doing two things to the clock. It is setting the time of the clock and it is changing the amplitude or strength of the clock."

As our circadian rhythm changes melatonin levels, we can use the levels of this 'sleep hormone' to see what is affecting our body clock. Several studies have shown that bright, artificial light suppresses melatonin production in humans.

Interestingly, very bright artificial light is actually used as a therapy (called phototherapy) to help people who have very delayed biological clocks wake up and go to sleep earlier.

The intensity of light used for phototherapy is much higher than what is emitted by any screens or light bulbs we use. A 2014 study looked at a more realistic scenario: comparing the melatonin levels and sleep quality of people who either read a normal book or an electronic book before bed. They found that the participants who read the electronic book had reduced melatonin levels.

Dr Cele Richardson from Western Australia University says "There is evidence that 1.5 hours (or more) of bright screen use reduces the natural nighttime increase in melatonin, and this effect may compound over multiple nights."

Importantly, she adds "however, this does not appear to translate to taking longer to fall asleep."

What does this mean for our sleep patterns?

Whilst we know that melatonin has many effects in the body and is associated with the sleep-wake cycle, we don't know exactly how reduced amounts of melatonin impact our quality of sleep.

There are numerous studies that look at technology use and sleep quality or the time it takes to fall asleep. Although many of these do find a correlation between screen time and sleep, the correlations are often weak and they don't show that increased screen time causes problems sleeping.

For example, the 2014 study found that on average participants who read the printed books fell asleep 10 minutes before the e-book readers. Other studies compared people who used products that reduced the blue light from screens to normal screen users. These studies found only a 3-4 minute difference in the time it took to fall asleep.

As sleep is affected by many things, it is often difficult to make sure it is just the effect of screen time that you are measuring.

Another complication is highlighted by Dr Richardson: "A bi-directional relationship between technology use and sleep is likely. That is, technology use may affect sleep over time, yet individuals who have trouble sleeping may subsequently increase their technology use."

The takeaway

Technology, specifically artificial light, does change our circadian rhythm. We know this because we can see differences in melatonin levels after screen use.

What effect this has on our sleep, particularly the time it takes to fall asleep, is not yet clear.

Article based on 4 expert answers to this question: Is technology changing our circadian rhythm?

This expert response was published in partnership with independent fact-checking platform Metafact.io. Subscribe to their weekly newsletter here.