UC Berkeley study links brain atrophy of aging with both poor sleep and poor memory retention. But the question is, does the atrophy lead to both poor sleep and poor memory directly, or does the poor sleep interfere with long-term memory storage (as it does at any age). A key paragraph:
Walker likened the waves to a chairlift that picks up new memories from the hippocampus – which can hold memories only temporarily – and escorts them to the prefrontal cortex, where they are cemented into the brains architecture. Brain scans during the morning test showed that the younger participants were relying on the cortex to recall the word pairs while the older people were still counting on the hippocampus.