Screwtape explains the law of undulation:
Humans are amphibians – half spirit and half animal. (The Enemy’s determination to produce such a revolting hybrid was one of the things that determined Our Father to withdraw his support from Him.) As spirits they belong to the eternal world, but as animals they inhabit time. This means that while their spirit can be directed to an eternal object, their bodies, passions, and imaginations are in continual change, for to be in time means to change.
Their nearest approach to constancy, therefore, is undulation – the repeated return to a level from which they repeatedly fall back, a series of troughs and peaks. If you had watched your patient carefully you would have seen this undulation in every department of his life – his interest in his work, his affection for his friends, his physical appetites, all go up and down.
As long as he lives on earth periods of emotional and bodily richness and liveliness will alternate with periods of numbness and poverty. The dryness and dullness through which your patient is now going are not, as you fondly suppose, your workmanship; they are merely a natural phenomenon which will do us no good unless you make a good use of it.