Depression, in many ways, is a situation in which the brain becomes untethered from its positive reward systems. It's not just absence of happiness or the presence of unhappiness, it's largely a loss of the desire be happy.

In this situation, the brain function in "low power mode". How much effort your brain expends on processing information is proportional to a variety of complex brain systems that measure how worthwhile what you're doing is. Doing something worthwhile results in positive reward.

No positive reward systems, no ability to detect the worthwhilness of activities, no proportional increase in effort expended.

Turns out, remembering, reading, understanding, doing math, general normal function, requires a lot of effort. If you're sufficiently depressed, it can be more ffort than the brain is calibrated to expend.

Keep in mind, there is a lot more to depression than I explained. For example, the serotonin system is just one of many system that calibrates threat sensitivity, mood stability, and as a result partially mediates systems like aggression and exploratory behavior. But that's not the whole picture. The important thing to understand is that the brain more or less shuts down