Studies have shown melatonin production is decreased by light, and when light is present in the room before bedtime, when it would normally be dark, the overall amount of melatonin produced is significantly reduced.

The intensity of light matters too, so it's not an all or nothing situation. Less light leads to less reduction of melatonin.

Since melatonin is a necessary component for a healthy brain and cardiovascular system, reduces risk of type 2 diabetes and cancer, those interested in staying up late might want to consider making some changes to screen brightness, buying glasses to reduce light intensity, and turning off room lights.

Some might be tempted to disbelieve this stressful information. Since stress is also damaging to the health, it might not be a bad decision. But one should consider how easy it might be to make some of these changes first.

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Interesting excerpts from studies:

Exposure to Room Light before Bedtime Suppresses Melatonin Onset and Shortens Melatonin Duration in Humans[1]

"Given that melatonin receptor genes have recently been linked to the pathogenesis of type 2 diabetes (18,–20, 51), it is possible that disruption of melatonin signaling by exposure to light at night could contribute to the increased risk for developing metabolic syndrome and type 2 diabetes"[1]

"Our results demonstrate that the melatonin profile is truncated by exposure to room light before bedtime. Specifically, we show that exposure to room light (<200 lux) in the late evening suppresses the onset of melatonin synthesis, thereby shortening melatonin duration by about 90 min compared with exposure to dim light (<3 lux). As a result of this exposure to electrical light between dusk and bedtime, presleep levels of melatonin were reduced by 71.4% and total daily levels of melatonin were reduced by about 12.5%. When room light exposure continues for the entire night, total daily melatonin is suppressed by more than 50% in most individuals, with median suppression of 73.7%."[1]

"Five intensities of artificial light were examined for the effect on nocturnal melatonin concentrations. Maximum suppression of melatonin following 1 hr of light at midnight was 71%, 67%, 44%, 38%, and 16% with intensities of 3,000, 1,000, 500, 350, and 200 lux (lx), respectively."[2]

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3047226/

[2] https://pubmed.ncbi.nlm.nih.gov/2915324/