Source: http://examine.com/supplements/Melatonin/#summary15-1
It appears that there's not much of a tolerance effect when taking melatonin. This is unusual, our experience tells us that most drugs have a tolerance effect. Extended use of caffeine causes the body to make more adenosine receptors, meaning that it takes a higher dose to have the same effect.
This does not appear to happen with melatonin. Less science and more anecdote from a lot of users: melatonin also has an amount where it is less effective. You'll find several reports of people that see no effect at the higher doses (10mg), yet a lower dose (3mg) has a good effect.
This is all moot if you're in the UK or Australia, since melatonin is a prescription drug.
Unfortunately whenever i get too hot, my body's way of self regulating is apparently to wake me up with a nightmare. Happens consistently in fact.
* Melatonin -- get the low dose chewable kind from Trader Joes and take 30 minutes before you go to bed
* Keep the bedroom cool and completely dark -- there is research the connects insomnia with warm body temperatures in some people (http://www.ncbi.nlm.nih.gov/pubmed/18603220)