Toronto: For decades, scientists have fiercely debated whether rapid eye movement sleep -- the phase where dreams appear -- is directly involved in memory formation. Now, a study provides evidence that this is indeed the case. Poor sleep quality is increasingly associated with the onset of various brain disorders such as Alzheimer's and Parkinson's disease. Results from this study, published in the journal Science, suggest that disruption in this important phase of sleep may contribute directly to memory impairment observed in Alzheimer's disease, the researchers said. "We were able to prove for the first time that REM sleep is indeed critical for normal spatial memory formation in mice," said study co-author Sylvain Williams, professor at McGill University in Montreal, Canada. This phase of sleep is understood to be a critical component of sleep in all mammals, including humans. For the study, the researchers used optogenetics, a recently developed technology that enables scientists to target precisely a number of neurons and control their activity by light. "We chose to target neurons that regulate the activity of the hippocampus, a structure that is critical for memory formation during wakefulness and is known as the 'GPS system' of the brain," Williams said. To test the long-term spatial memory of mice, the scientists trained the rodents to spot a new object placed in a controlled environment where two objects of similar shape and volume stand. Mice spend more time exploring a novel object than a familiar one, showing their use of learning and recall. When these mice were in REM sleep, however, the researchers used light pulses to turn off their memory-associated neurons to determine if it affects their memory consolidation. The next day, the same rodents did not succeed the memory task learned on the previous day. Compared to the control group, their memory seemed erased, or at least impaired. "Silencing the same neurons for similar durations outside REM episodes had no effect on memory. This indicates that neuronal activity specifically during REM sleep is required for normal memory consolidation," study's lead author Richard Boyce from McGill University noted.