It is based in part on some molecular evidence as well as on measurements of so-called slow waves in the brain, which occur during non-REM sleep and can be thought of as a marker of synaptic strength. Tononi has shown that if research subjects tax a particular brain area during the day—for instance, by performing an experimental learning task—their slow waves in that area may be larger when they sleep. Conversely, if subjects' use of a particular brain area is restricted during the day, the slow waves in that area may be muted during sleep. Slow waves also seem to get smaller over the course of the night, suggesting that synaptic strength overall may be decreasing. Tononi is currently measuring synaptic strength before and after sleep—which may show more directly how the strength of the wiring is changing. But his results are not yet available. (For a more technical discussion of the hypothesis, click here.)