My friend used to have two ‘days’ each day, with a nap between—in the afternoon, he would get up and plan his day with optimism, whatever happened a few hours before washed away. Another friend recently suggested to me thinking of the whole of your life as one long day, with death on the agenda very late this evening. I used to worry, when I was very young, that if I didn’t sleep, I would get stuck in yesterday forever, while everyone else moved on to the new day. Right now, indeed some people have moved on to Monday, but I’m still winding down Sunday because I had a bad headache and couldn’t sleep. Which is all to say, a ‘day’ does not just mean a 24 hour measure of time, in our minds. Among its further significance, we treat it as a modular unit: we expect things within it to be more continuous and intermingled with each other than they are with things outside of it. What happens later today is more of a going concern at present than something that happens after sleeping. The events of this morning are more part of a continuous chapter, expected to flavor the present, than what happened yesterday. The same is true to some extent for weeks, months and years (but not for fortnights or periods of 105 hours).

I think days are well treated as modular like this because sleeping really separates them in relevant ways. I notice two other kinds of natural modular time-chunks that seem worth thinking in terms of, but which I don’t have good names for:

• Periods during which you are in one context and stream of thought (usually a minute to a few hours long). For instance the period of going for a walk, or the period between getting home and receiving a phone call that throws you into a new context and set of thoughts. During one such chunk, I can remember a lot about the series of thoughts so far, and build upon them. Whereas if I try to go back to them later, they are hard to bring back to life, especially the whole set of thoughts and feelings that I wandered around during a period, rather than just a single insight brought from it. Within chunks like this, my experience seems more continuous and intermingled with other experience within the chunk. Then I get an engaging message or decide to go out, and a new miniature chapter begins, with new feelings and thoughts. (Though I’m not sure how much other people’s thoughts depend on their surroundings, so maybe for others a change of context is less of a reset).
• Similarly, longer periods of repeatedly being in particular places with particular people. These might be decades of settled marriage or a few days of being on a trip. For me they are often a month to a year. They are punctuated by moving, breaking up, changing jobs. They tend to have their own routines and systems and patterns of thought. For me, starting a new one is often marked by a similar optimism and ambition for a fresh start as mornings. And ending one shares with evenings a risk of sadness at wasted opportunity.

Both of these also end because of something like sleep—changes of context that break the continuity of thoughts or habits within the period, either because those things relied on the previous context as something like memory, or because the new context asks for a new activity that replaces the old one, and the old one needed the continuity to stay alive.