Blue light from screens in the evening disrupts your biological clock through specialised retinal cells and suppresses the production of melatonin. There are indications that reducing blue light measurably improves sleep quality, but the evidence comes primarily from small studies with self-reported outcomes.
Evening screen light, and especially the blue part of the light spectrum, disrupts your biological clock. Your retina contains special light-sensitive cells (melanopsin ganglion cells) that are highly sensitive to blue light. When these cells are stimulated in the evening, they send a signal to your brain that it is still daytime, which suppresses the production of the sleep hormone melatonin and throws your circadian rhythm (the day-night cycle) out of sync. This is the biological mechanism behind the negative effects of screen light on sleep (PMID 36744480).
There is also direct evidence that reducing blue light in the evening concretely improves sleep quality. In a study of medical students (aged 20-22), the sleep quality score (measured with the PSQI, where a lower score is better) fell from 6.83 to 3.93 after they reduced the blue light from their smartphones. That is a strikingly large difference: from 'poor' to 'good' sleep quality, with improvements in daily functioning as well (PMID 36744480). A note of caution: this involves a small, specific group of young adults with self-reported outcomes, so this effect cannot simply be generalised to everyone.
The reverse situation, namely using light therapy to treat insomnia, illustrates how complex the relationship between light and sleep is. European guidelines for the treatment of sleep problems do mention light therapy as a possible non-pharmacological option, but at the same time conclude that there is insufficient good evidence to make strong recommendations about it. It is therefore not an established therapy for chronic insomnia (PMID 28875581, 36503403).
In summary: the harmful effect of blue screen light on your biological clock is well supported biologically, and there are indications that limiting it in the evening measurably improves sleep quality. However, the strength of the evidence remains limited due to small and specific study populations and self-reported outcomes. The severity of the effect will vary from person to person, but the mechanism is real enough to warrant caution about screen use just before bedtime.
The biological underpinning (melanopsin cells, circadian rhythm) and the student study both come from PMID 36744480. The evidence for light therapy in insomnia is insufficient (PMID 28875581, 36503403). No large RCTs or meta-analyses on evening screen light are available in the cited claims.