As one stays awake for a long time, so much extra potentiation has already happened from the waking day, and trying to force more LTP isn't going to help anything. Too much extra information is floating around, and the neurons can't handle all of the extra activity. In addition, however, as the night starts to loom, the circadian rhythm begins to take effect and the body naturally begins to release stores of melatonin. This increase in melatonin reduces the ability to learn and facilitate new memories. The ability for melatonin to suppress memory formation is very significant, however. The melatonin would work in conjunction with the LTD during slow oscillations during sleep, to keep individuals from potentiating unwanted, or unneeded, information from their day.