When do human beings start to dream?

Taken from Scientific American Magazine.

Paul Li, lecturer of cognitive science at the University of California, Berkeley, replies: Pinpointing when humans begin to dream remains an elusive challenge, although scientists have some ideas. There are researchers who argue that dreams originate as early as in the mother’s womb, whereas others posit that they first occur when a child’s brain becomes more developed, around five to seven years old.

Self-reports of dreams provide the only reliable evidence that a person can dream. Unfortunately, it is impossible to ask a newborn infant or a fetus whether it had a dream last night. Instead scientists can gather clues about when we begin to dream by monitoring certain physiological markers while a person is asleep, such as brain waves, muscle tension and eye movements.

One stage of sleep, in particular, often indicates when a person is dreaming. This stage, called rapid eye movement, or REM sleep, typically occupies about 20 percent of an adult’s night sleep. Newborn babies may spend more than 80 percent of their total sleep time in REM.

Fetuses also experience REM sleep. Studies using ultrasound have shown that fetuses exhibit REM sleep as early as the 23rd week of gestation.

Although scientists can detect REM activity in fetuses, they cannot know for certain whether this physiological activity, specifically eye movements, indicates that the fetus is dreaming. This inability to determine what is happening is because humans do not necessarily always dream during REM sleep, and humans can dream outside of this sleep stage.

But even if we could assume, for a moment, that fetuses dream, what would they imagine in their sleep? And how much would their dreams differ from those children and adults have? These questions are certainly worth sleeping on.