There is a well-warn platitude that “debt is stealing from your future self.” The logic follows that instead of (or worse, in addition to) spending the resources you currently have, debt is the act of spending against resources you will have down the road. Debt functions on highly optimistic principals, namely that future you will be so very well off that not only will that version of yourself have enough means to cover your then-current expenses, but also surplus to cover the now-current expenses present-day you is unwilling or unable to afford.
In terms of productivity, mental capacity and energy, caffeine is debt in liquid form. We tend to think of coffee and espresso shots as a fuel or energy source - something additive that picks us up, makes us faster/sharper/more alert. In truth caffeine is reductive, functioning not by adding energy but by silencing the brain’s exhaustion signals, muting the alarms that our brains and bodies need rest in a form of biological debt, one that will need to be repaid.* The compound adenosine serves as the biological sand in our metaphorical hourglass, collecting in our nervous systems during waking hours until we sleep and the chemical clock is reset. Adenosine is our chemical gas gauge, letting us know when we are mentally running on fumes. Enter caffeine; chemically able to bind to adenosine receptors, caffeine acts to inhibit our ability to accurately gauge fatigue. This is tantamount to covering your car’s temperature warning light with magic marker when your engine is overheating. This overloading act has similar effects as any form of debt. By taking possession of energy and alertness that does not belong to present-day you (but is the rightful property of a future version of yourself), you make the assumption that future you will have ample energy and rest to spare. Sadly this is rarely the case; sometimes we can steal back rest on weekends or crash for a day after a hard-burn, but more often than not, the future version of us is also amassing debt in the form of late nights and lattes. The cyclical levying of energy tax on our future selves accrues interest, as we are not actually performing at peak during those stolen hours. We are tired, even if we have fooled our bodies’ analytics into thinking otherwise. Our ability to reason and execute is compromised at a biological level - that is the reason our adenosine fuel gauge was installed in the first place. So each day we wake, burdened with the debt assigned by past-us and the sub-par work produced by a compromised mind and body, and we double down with more shots in our venti red eye.
Debt can be a strategic tool in moments when it is legitimately viable that future you will have significantly more resources than current you. A business with massive (recoverable) accounts receivables but little cash-on-hand can leverage debt to bridge the gap. But as a sea of failed zero-rate startups learned the hard way, accruing debt against “likely future success” is a dangerous game.
*yes, technically caffeine acts as a mild stimulant in addition to the aforementioned fatigue-blocking properties. While not totally inconsequential, there are plenty of other mild stimulants that draw comparatively little interest: we don’t see the world consuming 1.2 tons of ginseng or yerba mate a year, and there’s a reason for that.