Is it a good idea to study new words before bed? Scientists in the UK tested this. People learned new made-up words. One group learned them at 8 p.m. and then slept. After one night, the new words had joined their mental dictionary.

Another group learned at 8 a.m. and stayed awake all day. Their words did not change until after they slept. So study words in the evening, and then go to sleep.

Many students believe that what you study before bed is easier to remember. Research on vocabulary supports this idea. In 2007, researchers at the University of York taught people made-up words similar to real ones, such as "cathedruke" for "cathedral." If a new word really enters the mental dictionary, it should compete with the real word and slow down recognition of it.

People who learned the words in the evening showed this competition after one night of sleep. People who learned in the morning showed no change after twelve hours awake, and only showed it after sleeping. A German study found that high school students remembered foreign words better when they slept within a few hours of learning. Losing sleep, on the other hand, harmed memory.

Is "cathedruke" a word? No, but Gaskell and colleagues at the University of York taught invented words like it to volunteers for a reason. If a made-up word such as "cathedruke" has truly joined the mental lexicon, it should start competing with its real neighbor "cathedral" and slightly slow its recognition. In a 2007 study, participants who learned novel words at 8 p.m. showed no competition right away, but they did twelve hours later, after a night's sleep.

Participants who learned at 8 a.m. showed no change after twelve waking hours and showed competition only after they had slept. Time alone was not enough. Sleep made the difference. A 2010 study linked this integration to sleep spindles, short bursts of brain activity during non-REM sleep. The spindles predicted how well the new words fit in with existing vocabulary, though not how many words people could recall.

Most of this evidence comes from laboratory experiments with made-up words, and playing words to people during sleep only strengthens items already learned while awake. Applied research points the same way, though. High school students recalled foreign vocabulary better when they slept within a few hours of learning, and losing sleep caused lasting harm.