We’ve probably all experienced how a poor night’s sleep can make us feel tired, irritable and have difficulty concentrating the next day. But while the odd night of poor sleep has no impact on our health, research shows that prolonged sleep disturbances predict cognitive decline – and are also a risk factor for dementia. Disrupted sleep is also known to be a symptom of Alzheimer’s disease, a type of dementia.
But while we know that poor sleep is connected to dementia and Alzheimer’s disease on a long-term scale, until now it was unknown to what extent night-to-night variation of sleep affects dementia symptoms in the short term (such as the following day). This is something we tried to answer in our recently published study.
We found that nightly variations in sleep (such as sleeping too long or waking up in the night) had a greater effect on some aspects of brain function (such as memory and mood) the next day in people with Alzheimer’s disease compared to those with mild cognitive impairment or no cognitive impairment.
To conduct our study, we looked at 15 participants with Alzheimer’s, eight with mild cognitive impairment and 22 with no signs of cognitive impairment to compare the relationship between sleep and daytime function.
For two weeks, participants reported their sleep quality and how long they slept using a sleep diary. We also used an activity monitor to record objective sleep measures such as how long participants slept during the night or how long it took them to fall asleep.
To see whether the previous night’s sleep had an effect on their cognitive ability, we also phoned participants every morning to test things like their thinking ability and memory. For example, participants were asked to count backwards in sevens (calculation ability), or to recall a list of words (memory).
In addition to this, participants completed daily measures of their mood (such as how alert they were feeling) and whether they’d experienced any memory problems (such as forgetting an appointment) during a daily telephone session. To ensure that none of the participants with cognitive impairment or Alzheimer’s forgot to complete these tasks, we invited caregivers to help remind them. Caregivers also documented participants’ daily patterns of behaviour.
We found that greater sleep continuity (waking up fewer times during the night) was generally better for daytime performance. Participants with Alzheimer’s had improved alertness the next evening and made fewer memory errors during the day. Both participants with Alzheimer’s and mild cognitive impairment also had fewer observable behavioural problems (such as crying, aggression or asking repeated questions) the next day following higher sleep continuity.
Surprisingly, we also found that for all participants, regardless of whether they had cognitive impairment or not, greater sleep continuity was actually related to worse calculation ability the next day.
These findings persisted even when we adjusted for other factors that might impact results – such as sex, age and years of education. We also excluded participants with conditions, such as anxiety, depression and sleep disorders, which affect sleep and cognitive ability, and thus could have influenced the results.