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.