Found it:

Originally posted by EWLD
Q. Wont all these efforts and exercises for becoming lucid lead to loss of sleep? And wont I feel more tired after being awake in my dreams? Is it worth sacrificing my alertness in the daytime just to have more lucid dreams?

A. Dreaming lucidly is usually just as restful as dreaming nonlucidly. Since lucid dreams tend to be positive experiences, you may actually feel invigorated after them. How tire you feel after a dream depends what you did in the dream-if you battled endlessly and nonlucidly with frustrating situations, you will probably feel more tired than if you realised in the dream that it was a dream and that none of your mundane concerns were relevant .
* * You should work on learning lucid dreaming when you have time and energy to devote to the task. The exercises for increasing dream recall and inducing lucid dreams probably will require that you spend more time awake during the night that usual, and possibly that you sleep longer hours. If you are too busy to allot more time to sleeping or sacrifice any of the little sleep you are getting, its probably not a good idea for you to work on lucid dreaming right now. Doing so will add to your current stress, and you probably won't get very good results. Lucid dreaming, at least at first, requires good sleep and mental energy for concentration. Once you learn the techniques, you should be ableto get to at point at which you can have lucid dreams at any time you wish just by reminding yourself that you can do so.