DLMO: the body's clock-time marker
The dim-light melatonin onset, or DLMO, is the point in the evening when melatonin first rises under low-light conditions. Researchers use it as a reliable marker of internal circadian time, because it is more stable than asking when someone feels sleepy. Where the DLMO sits tells you whether a person's clock is early, typical, or late.
Evening exposure to bright or blue-enriched light can delay the DLMO, meaning the melatonin rise that should mark the start of biological night is pushed later into the night.
Delay plus a smaller peak
Strong evening light does two things at once: it shifts the timing of melatonin later and it can lower the height of the night-time peak. The combination means the signal for sleep arrives later and is weaker, which is a plausible route from late-night screen use to delayed sleep timing.
The size of the delay depends on light intensity, color, duration, and personal sensitivity, so reported ranges are approximate. This is general educational information about circadian biology, not medical advice.