We know circadian rhythm affects learning and memory in that we find it easier to learn at certain times of day than others, but now a study involving Siberian hamsters has revealed that having a functioning circadian system is in itself critical to being able to remember. The finding has implications for disorders such as Down syndrome and Alzheimer's disease. The critical factor appears to be the amount of the neurotransmitter GABA, which acts to inhibit brain activity. The circadian clock controls the daily cycle of sleep and wakefulness by inhibiting different parts of the brain by releasing GABA. It seems that if it’s not working right, if the hippocampus is overly inhibited by too much GABA, then the circuits responsible for memory storage don't function properly. The effect could be fixed by giving a GABA antagonist, which blocks GABA from binding to synapses. Recent mouse studies have also demonstrated that mice with symptoms of Down syndrome and Alzheimer's also show improved learning and memory when given the same GABA antagonist. The findings may also have implications for general age-related cognitive decline, because age brings about a degradation in the circadian system. It’s also worth noting that the hamsters' circadian systems were put out of commission by manipulating the hamsters' exposure to light, in a technique that was compared to "sending them west three time zones." The effect was independent of sleep duration.
Circadian clock may be critical for remembering what you learn
 Hippocampal-dependent learning requires a functional circadian system. Proceedings of the National Academy of Sciences. 105(40), 15593 - 15598.(2008).