Rapid Eye Movement. The phase of sleep in which you dream.

https://en.m.wikipedia.org/wiki/R.E.M.

Think of your sleep as reading a series of engaging books, where each book represents a sleep cycle, including chapters of both deep sleep (SWS) and dream sleep (REM). When you finish a book (sleep cycle), you reach a satisfying conclusion to that story arc—this is akin to waking up after a full sleep cycle. You feel refreshed because you’ve concluded the narrative neatly, without interrupting a tense plot twist or leaving a storyline unresolved.

However, just finishing one book doesn’t mean you’ve completed the whole series. If you stop after one book each night, you’re missing out on the depth and development that comes from reading more of the series (accumulating more sleep cycles). Initially, you might feel okay because you’ve concluded a story (cycle) properly, avoiding the grogginess of waking up mid-chapter (mid-cycle). Yet, this approach doesn’t give you the full, enriching experience (or rest) your body and brain need over time.

As days go on, if you continue this pattern, you accumulate a ‘reading debt’—akin to sleep debt. You’ve missed out on the broader, deeper insights and the full narrative arc that only comes from reading (sleeping) the whole series or book. This debt reflects not fully recharging your brain and body, leaving you progressively more tired. While you might feel a temporary refreshment from completing a cycle, without the full, restorative rest of multiple cycles, you’re not truly at 100%—you’re running on the satisfaction of a finished story, not the full restoration that comes from a complete series.

Sleep apnea means your airway narrows enough while you sleep that you aren't getting enough air, which leaves you exhausted after struggling to breathe all night