A few supplements have a real evidence base for sleep, and a great deal of marketing piled on top. The honest picture is that the better-supported ones tend to help at the edges, with onset or quality, rather than knock you out the way a sleeping pill would. None of them substitutes for the basics: a consistent wake time, a dark cool room, and getting off screens and caffeine earlier than feels necessary.
Magnesium
Magnesium, especially the glycinate form, is a low-risk starting point with modest evidence. In one placebo-controlled trial in older adults with insomnia, at 500mg a day (above the dose we recommend), people rated their sleep as better, though total time asleep did not change. The safety margin is wide, and it may be a reasonable first thing to try, taken in the evening.
Glycine
Glycine, taken as a few grams before bed, has small trials, mostly from one research group, in which people felt more rested and performed better the next day. It may work partly by lowering core body temperature, which is part of how the body settles into sleep. It is cheap, well tolerated, and may be worth a try if magnesium alone is not enough.
Melatonin (a UK note)
Melatonin is worth understanding carefully. In the UK it is a prescription medicine, not an over-the-counter supplement as it is in the US. Its evidence is strongest for problems with timing: jet lag, shift work, and delayed sleep phase, where it helps shift the body clock. For ordinary "I can't switch off" insomnia its effect is smaller than its reputation suggests. It is best thought of as a body-clock tool rather than a sedative.
L-theanine
L-theanine, an amino acid found in tea, has trials whose pooled effects on falling asleep are small and self-rated. It is very safe, and some people find it helpful for winding down rather than for sleep itself.
The honest version
Start with the basics and magnesium, add glycine if you need more, treat melatonin as a timing tool, and keep expectations realistic about the rest. If your sleep problem is really a stress problem in disguise, that may be the better place to start. To check any sleep supplement against a medication you take, the free checker will show you the result and the reasoning.