A hundred years ago, these naval aviators look like they’re sleeping on duty, but sleeping was their duty. They are busy learning Morse code, and the Navy discovered that they could do so by sleep learning. While they were asleep, messages were sent to them at increasingly higher speeds. It was found that when they woke up, they were able to copy that speed. In fact, some of the trainees were able to recite the messages that had been sent to them while they slumbered.
The photo appeared in the December, 1923, issue of Popular Mechanics.
This is very true.
55 years ago.when I was a Child my old man wanted me to learn morse code and besides to send me out to a CW local académico, he run while I was sleeping a recorded tape for me to practice improving my code skills and when I woked up I could remember all the words he sent in the recording.
Today I am a fast professional morse code thanks to those recordings been able to copy by ears up to 70 wpm.