Read the full article at Quartz.

Think about the last time you were really happy. Not just when you were pleasantly surprised, or you had a “nice” time, but rather a moment in which you were smiling uncontrollably and laughing not because someone said something funny, but because it was a moment of pure bliss.

Those moments tend to be few and far in between (it’s well worth writing them down so you start to recognize what leads to them), and although they give us some of our fondest memories, they also tend to come with a downside: a dull feeling of gloom once they’re over.

For as long as I can remember, I’ve called these “happiness hangovers” (although I’m fairly certain I didn’t come up with the term on my own; I think I picked it up from a friend). These hangovers are not depression, but a temporary feeling that reality is a little greyer than usual. For me, regular routines feel like a disappointment, and I tend to dwell on the contrast between the happiness of the past and the bland present.

There isn’t a technical scientific term for this feeling, but it’s something almost all of us experience to some degree. Most likely, it a consequence of the way that humans experience pleasure.

Pleasure is an evolutionary gift. It’s usually life-affirming, which is why we feel it through things like sex and sustenance. Because humans have evolved to be more complex than replicating eating-machines, we also get pleasure from activities that involve a good degree of higher-order thinking, like spending time with loved ones, going to concerts, or experiencing nature—which in turn, leads to happiness. To the best degree of scientific understanding, all animals can experience pleasure but only humans can experience happiness.

Neurologically, pleasure comes from specific areas in the brain called hedonic hotspots. “We know of about five” in the human brain says Kent Berridge, a neuroscientist at the University of Michigan. As Berridge explains for Scientific American, when hedonic hotspots pick up signals that we’re experiencing something pleasurable, they release their own drug-like neurotransmitters. Nearby receptors pick up these neurotransmitters and create a sensation of liking. Simultaneously, hotspots work with other parts of the brain to coordinate wanting, which is triggered by the neurotransmitter dopamine. Out of that, we develop a conscious understanding that whatever we’re experiencing is pleasurable. Taken together, the system gives us a feeling of enjoyment and deep desire to keep that feeling going, or to get it again in the future.

How exactly these hotspots turn on and off isn’t fully understood, but Berridge thinks it makes sense that they’d be related to happiness highs and the lows that follow. Happiness, he says, is a part of pleasure, and pleasure is something we only feel at certain times.