1. Couches are quite dirty
Couches are seldom properly cleaned. Those throw pillows? But we clean our sheets at least once a week, which removes mites and bacteria, not to mention the fact that the smell is more relaxing; sleeping on clean sheets is one of life's pleasures.
2. The conditions are different
Sleep is best when it's part of a routine. Typically, the circumstances for couch sleeping also include television, which has a bright blue light that mimics sunlight. You may think you're sleeping while the TV is on, but you're only skimming the surface, and not going deep to achieve the real rest your body needs.
Also, sleeping on couches could mean you're sleeping in a new place (a fight with the missus, perhaps?), and both conditions have also been scientifically proven to deny your body deep sleep because you're counting predators instead of sheep. So while the occasional day-nap on the couch is well-advised, do your best not to lay down your head there for the duration.
3. You're in a room full of blue light.
Between your TV, the cable box, and our omnipresent cell phone, napping in there is all but shining a flashlight in your eyes.
Okay, that's a little hyperbolic, but surrounding yourself with electronics is not setting yourself up for good sleep: the sources of artificial light in the average home quadrupled over the past 50 years, while sleep deficiency increased alongside, and studies actually show that the longer you spend around screens, the harder it may be to fall asleep.
In short, sleeping in rooms that aren't entirely dark can profoundly mess with your internal clock.