found that learning followed by the first half
of the night's sleep led to much better memory of neutral material than did learning
followed by the second half ofthe night's sleep. The second-half condition was superior
to a retention interval of wakefulness. These experimenters concluded that stage-4
sleep following learning was more beneficial to memory than REM sleep following
learning. Ekstrand et al. (19) argued that this difference more strongly supports the
Decay Theory than the Interference Theory of Forgetting since interfering learning
should be absent in both stages of sleep, but the decay of memory should be more
rapid when physiological arousal is higher, in REM sleep.