Sleep disturbances are prevalent in patients with Alzheimer’s disease (AD), being one of the most troubling symptoms during the progression of disease. However, little research has been made to determine if impaired sleep patterns appear years before AD diagnosis. This study tries to shed light on this issue by performing polysomnographic recordings in healthy elders and patients with mild cognitive impairment (MCI). We further investigated whether changes in sleep patterns parallel memory decline as well as its relationship with the Apolipoprotein E (ApoE) ε 4 genotype. Results showed a significant shortening of rapid eye movement (REM) sleep together with increased fragmentations of slow-wave sleep in MCI patients relative to healthy elders. Interestingly, we further showed that reduction of REM sleep in MCI patients with ApoE ε 4 was more noticeable than in ε 4 non-carriers. Contrary to our initial hypothesis, changes in sleep patterns were not correlated with memory performance in MCI patients. Instead, increased REM sleep accompanied enhanced immediate recall in MCI ε 4 non-carriers. Taken together, these results suggest that sleep disruptions are evident years before diagnosis of AD, which may have implications for early detection of dementia and/or therapeutic management of sleep complaints in MCI patients.