How long a person with dementia can safely be left alone depends entirely on their stage and individual circumstances. In the very early stages, short periods alone may be manageable with the right safety measures in place. As dementia progresses, the risks increase rapidly. The person may forget to turn off appliances, become disoriented, fail to eat or drink, or be unable to summon help if they fall. Once someone is in the moderate stages, leaving them alone for more than brief periods is unlikely to be appropriate without professional risk assessment and mitigation measures in place.