I've been taking 1.5 mg of melatonin every night for the past two weeks (except for last night). It's hard to say exactly what effect it had, because it's confounded with another major change in my sleep habits. It seemed to me that it moved my energy levels towards "a little bit sluggish" relative to what I counterfactually expected had I not taken the melatonin. So if I had a good night's sleep, I seemed less energetic than I ought, and if I had very little sleep, I seemed more alert than I ought.

On Sunday night (Jan 17) , I got 5 hours of sleep and felt terrible on Monday. On Monday night I got 6.25 hours of sleep and felt very marginal on Tuesday. Last night I got 4.75 hours of sleep -- but I forgot to take the melatonin. Today I feel far better than I ought to on such little sleep. I'm going to neglect the melatonin again tonight and see what tomorrow brings.

ETA: Also, I noticed no change in the vividness or lucidity of my dreams relative to my baseline of rarely remembering any dreams for longer than 30 seconds after waking up.

I tried to get some just know. But it turns out in the UK you can only get melatonin on prescription.