Nights, natural nights without artificial lighting or brutal stimuli, are the space where our perceptions are challenged in a very different way. Our sight is impeded by the lack of light. The diurnal hysteria diminishes drastically. Perhaps our biological reactions are still and trigger reflexes that seem anachronistic. The skilled hunter becomes more vulnerable. Sleep deprives us of our defence mechanisms.

Disclaimer: Dismiss at your will. This is just my own guess. Not a theory. Not even a hypothesis.

If what you mean by "most difficult to get through" is either of these or some combination:

1. Emotionally upsetting
2. Physically demanding
3. Psychologically altering (in the sense that it is difficult to manage a particular state of mind - focus)

Then I would say That's it is because at night, you experience hormonal and other biochemical shifts as a result of a number of things following:

• a lack of exposure to sunlight
• lessened communication with other persons (because people tend to sleep at night and thus, are unable to interact)
• passing circadian rythims and other biological clock sects
• dinner
• extended periods of time being physically and mentally "inert" and so exploring a number of thoughts
• I could go on.

Sorry if your question went over my head and it might have been rhetorical, or demanding of some sort of emotional support. Thanks for reading though lol