At night there's generally more interference around - AM radio signals become stronger, street lights and other lighting increase, TVs get switched on, and so on. The reason these types of interference tend to affect the higher tones most is that these higher frequencies are more attenuated in the cable from the exchange to you.
Incidentally, when you get a crackly line fault, it's not uncommon for this to affect mainly the lower frequencies, because the crackles are predominantly low frequency. This type of fault can in some cases result in problems with the upstream part of the connection rather than the downstream - a problem I've encountered more than once.