Too little time in your single or double bed could contribute to Alzheimer's, a study has shown. But could too much sleep do the same? In a US study of over 15,000 women aged 70 and above, those who had five hours or less sleep each night didn’t do as well in mental tests compared to the seven-hour sleepers. However, the women who spent nine hours sleeping each night didn’t do as well either. Experts say that sleeping for longer or shorter than seven hours had the same effect as growing two years older. The study, which was carried out by Nurses Health and presented to the Alzheimer's Association International Conference in Canada, assessed the mental function of the women every alternate year for six years after they turned 70. Beforehand, however, researchers recorded the women's average sleep duration twice; once when they were aged between 40 and 65, and then when they were between 54 and 79 years old. The women whose sleep patterns changed by two hours or more as they went from mid to later life suffered more severe mental decline than the women who maintained a more constant sleeping pattern. The researchers also found evidence that sleep duration changed a molecular biomarker of Alzheimer's, indicating that too much or too little sleep could put a person at higher risk of the condition.