Drug info - cycles/cycling Supplements?

[Euphoric]

Would someone be so kind to explain the why's and how's of cycling (taking a supplement for a certain amount of time, and then refraining for a certain amount of time, repeat)? I've seen people reporting different cycles for different nootropics, but I don't understand why exactly (I'm pretty sure it has something to do with the tolerance and efficacy of the substance). A guide for cycling would be much appreciated, as sites and the containers for nootropics/herbal supplements don't seem to mention it, they just mention the daily dose.