Sleep and memory, why sleeping after you study matters

In 1924, Jenkins and Dallenbach compared two situations after learning. In one, participants slept. In the other, they stayed awake. There was less forgetting when people slept after learning than when they stayed awake (Jenkins and Dallenbach, 1924). It is one of the oldest experiments on the topic, and it stayed famous because its idea is simple.

This page is descriptive. It reports what researchers observed. It says that, within its experiment, what happens right after learning changes how much people forget. The next sections show what more recent work says about it.

Rasch and Born (2013) published a review in Physiological Reviews. Their starting point is that sleep takes part in the consolidation of memories (Rasch and Born, 2013). Consolidating means stabilising a trace that has just formed so that it lasts. The word is a bit technical, but a picture helps. A lesson that has just entered your head is still fragile, and what happens next matters.

The review brings a frame that fits the 1924 study, sleep is not lost time between two study sessions.

Murre and Dros (2015) repeated Ebbinghaus’s experiment on the forgetting curve, with a single subject and 70 hours of learning. They measured the savings in relearning after 20 minutes, 1 hour, 9 hours, 1 day, 2 days, 6 days and 31 days. Their result resembles Ebbinghaus’s, and the curve rises again at 24 hours. According to the authors, this is probably an effect of sleep (Murre and Dros, 2015).

Look at the table. After 9 hours, the saving is 27.6%, then it rises to 31.7% after 1 day, before falling again. That small bump is what the authors link to sleep. Be careful, a saving is not a percentage of memory. It is the share of relearning time you save because you had already seen the list. And it is a single subject, so it is not a rule for everyone. For the curve itself, see What Ebbinghaus’s forgetting curve really shows.

What happens between two sessions is not empty, and leaving a night between a first session and a recall is consistent with what these works describe.

For finals, an AP exam or a competitive test, this may simply mean that you do not try to do everything in a single evening. A session in the evening, a recall the next day, is a format that the guide Spaced repetition, how long to wait between two reviews presents with its own sources. If you have doubts about your sleep or your health, that is a topic for a professional, not for a revision page. Here the idea is modest. Learning has an aftermath, and it is part of the process.