Light timing decides the direction of the effect
Light does not simply reduce melatonin; when it occurs determines how it moves the clock. Light in the late evening and early night tends to delay the rhythm, pushing the melatonin rise and sleep onset later. Light in the late night and morning tends to advance the rhythm, pulling them earlier. This timing dependence is described by what researchers call a phase response curve.
Because evening is when most artificial light is encountered indoors, the practical result for many people is a tendency toward delay, with sleep drifting later than the body would otherwise choose.
Why intensity and wavelength matter
Brighter light and light richer in short blue wavelengths suppress melatonin more strongly, because the retinal cells driving the response are tuned toward blue. Dim, warm light late in the evening disturbs the signal less than bright cool light. Even so, individual sensitivity varies widely, so the same room affects two people differently.
This is general educational information about how light influences sleep timing, not medical advice; persistent sleep timing problems are best discussed with a qualified clinician.