Some researchers contend that sleep plays no role in the consolidation of declarative
memory (i.e., memory involving factual information). These researchers note that peo-
ple with impairments in rapid eye movement (REM) sleep continue to lead normal
lives, and they argue that if sleep were crucial for memory, then these individuals
would have apparent memory deficits. Yet the same researchers acknowledge that the
cognitive capacities of these individuals have never been systematically examined, nor
have they been the subject of studies of tasks on which performance reportedly
depends on sleep. Even if such studies were done, they could only clarify our under-
standing of the role of REM sleep, not sleep in general.
These researchers also claim that improvements of memory overnight can be
ex plained by the mere passage of time, rather than attributed to sleep. But recent
studies of memory performance after sleep — including one demonstrating that sleep
stabilizes declarative memories from future interference caused by mental activity
during wakefulness — make this claim unsustainable. Certainly there are memory
consolidation processes that occur across periods of wakefulness, some of which
neither depend on nor are enhanced by sleep. But when sleep is compared with wake-
fulness, and performance is better after sleep, then some benefit of sleep for memory
must be acknowledged.