Researchers in Germany have found that very immature sheep fetuses can enter a dreaming sleep-like state weeks before the first rapid eye movements are seen.
Mathematical analysis carried out by mathematician Karin Schwab and neuroscientists at Friedrich Schiller University in Jena provides a tool to better understand the purpose of sleep.
It may also be helpful in studying how the brain develops, and in identifying vulnerable periods in brain development when damage could lead to disease later in life.
The researchers point out that directly measuring the brain activity of a human fetus in the womb is impossible, and that whatever is known about the early sleep habits comes mostly from watching eye movements.
The first rapid eye movements are seen around the seventh month of a fetus' development.
For the current research, Schwab studied sheep, an animal that typically carries one or two fetuses similar in size and weight to a human fetus.
The course of brain development is also fairly similar in humans and sheep, lasting about 280 days in humans and 150 days in sheep.
The researchers recorded electrical activity in the brain of a 106-day-old developing sheep fetus directly, something that had never been done before.
Schwab's team used sophisticated mathematical techniques for detecting patterns, and found cycles in the complexity of immature brain activity.
Unlike sleep patterns in later stages of development, the cycles fluctuated every 5 to 10 minutes and changed slowly as the fetus grows.
The researchers concede that it is difficult to imagine what the fetus experiences during such cycles in terms familiar to adults, but add that the patterns shed new light on the origins of sleep.
"Sleep does not suddenly evolve from a resting brain. Sleep and sleep state changes are active regulated processes," says Schwab.
The new findings are also consistent with other data that shows that the brain cells that generate sleep states mature long before the rest of the brain is developed enough to fall into REM sleep.
Schwab says that a better understanding of brain development may provide clues about diseases later in life, like neurological disorders or crib death.
A research article on the mathematical analysis has been published in the journal Chaos.