... I have always found that the Trough periods of the human undulation provide excellent opportunity for all sensual temptations, particularly those of sex. This may surprise you, because, of course, there is more physical energy, and therefore more potential appetite, at the Peak periods; but you must remember that the powers of resistance are then also at their highest. The health and spirits which you... use in producing lust can also... be very easily used for work or play or thought or innocuous merriment. The attack has a much better chance of success when the man's whole inner world is drab and cold and empty. And it is also to be noted that the Trough sexuality is subtly different in quality from that of the Peak - much less likely to lead to... "being in love," much more easily drawn into perversions, much less... generous and imaginative and even spiritual... It is the same with other desires of the flesh. You are much more likely to make [a] man a sound drunkard by pressing drink on him as an anodyne when he is dull and weary... than... when he is happy...

The Screwtape Letters