The average person spends about two hours a night dreaming, adding up to five or six years of life. This episode starts with the word itself: in Old English, dream meant noise, joy or music, and only took on its sleep meaning in the 13th century under Old Norse influence. In Bronze Age Mesopotamia, Egypt and Greece people did not have dreams, they saw them, as visits from gods or ancestors. Around 2100 BCE, Gudea, king of Lagash, rebuilt the temple of Ningirsu because a dream figure told him to, while Greeks slept in sanctuaries hoping for visions and Hippocrates read dream stars as clues to illness.

Modern data is less romantic. Calvin S. Hall coded more than 50,000 dream reports, and anxiety is the top emotion, with sexual dreams under 10 percent. During the COVID-19 pandemic Deirdre Barrett found fear and illness themes running two to four times higher. The episode covers the left brain interpreter, why dream movements take 30 to 50 percent longer, and four competing theories of why we dream, before ending with the lucid dreamers and brain scanners now reading the dreaming mind.

• People said they were more likely to cancel a flight after dreaming of a crash than after merely thinking about one
• Francis Crick and Graeme Mitchison’s 1983 reverse learning theory and Antti Revonsuo’s threat simulation hypothesis
• Erik Hoel’s overfitting hypothesis, borrowed from how AI models are trained with noise
• David Eagleman and Don Vaughn’s idea that dreams keep the visual cortex from being taken over in the dark
• Keith Hearne and Alan Worsley’s 1975 eye signals, and researchers decoding the dream phrase I love you from muscle signals