Reptiles do experience sleep states that closely resemble the REM (rapid eye movement) phase associated with dreaming in humans. Whether they have subjective dream experiences, with images and narratives, remains unknown. But the brain activity patterns are strikingly similar to what happens in your brain when you dream, and the discovery has reshaped how scientists think about sleep across the animal kingdom.
What Happens in a Sleeping Reptile’s Brain
The breakthrough finding came from bearded dragons. Researchers at the Max Planck Institute recorded brain activity in sleeping lizards and found two distinct sleep states cycling back and forth: slow-wave sleep, marked by low-frequency, high-amplitude brain waves and sparse neuronal firing, and a REM-like state with brain activity that looked almost indistinguishable from wakefulness. During REM-like sleep, the dragons showed rapid eye movements, the hallmark physical sign that gives REM sleep its name in mammals.
These two states alternated with surprising regularity, cycling every 80 seconds for six to ten hours through the night. That’s dramatically faster than the human version. Your sleep cycles last about 90 minutes each, and you typically move through four or five per night. A bearded dragon can complete hundreds of these micro-cycles in a single sleep period.
The slow-wave patterns in the lizard brain closely resemble those recorded in the rodent hippocampus, a region critical for memory. But they originate from a completely different brain structure called the dorsal ventricular ridge, which has no direct anatomical equivalent in mammals. This means reptile brains arrived at a similar electrical pattern through a different physical architecture.
Not All Reptiles Sleep the Same Way
The bearded dragon’s neat 80-second cycle isn’t universal. Argentine tegus, a large lizard species from South America, also show two distinct sleep states, but without any clear periodic rhythm. Their first sleep state features high-amplitude sharp waves. Their second state shows 15-hertz oscillations (a frequency never previously recorded during sleep in any species), isolated eye movements, and drops in heart rate variability and muscle tone.
That second state shares key features with mammalian REM sleep: eye movements occur, and it can be suppressed by drugs that increase serotonin activity, just as REM sleep can be suppressed in birds and mammals. But the tegu’s brain waves during this state don’t look like the “desynchronized” pattern seen in dreaming mammals, bearded dragons, or birds. It’s recognizably related to REM, but it’s also clearly its own thing.
A 2025 study expanded the picture further by recording brain activity across seven lizard species, plus humans, rats, and pigeons. All of them shared a slow brain rhythm during sleep that was tightly linked to eye movements, muscle tone, heart rate, and breathing rate. In chameleons, this rhythm even correlated with changes in skin brightness during sleep. The finding suggests this underlying sleep rhythm is conserved across all amniotes, the evolutionary group that includes reptiles, birds, and mammals.
The Brain Regions That Control Reptile Sleep
The reptilian cortex has a clear three-layered structure made up of diverse neuron types, similar in organization to the mammalian cortex. A region called the anterior dorsal cortex is considered the likely equivalent of the human neocortex, the outer brain layer responsible for complex thought and sensory processing.
When researchers surgically removed the cortex on both sides of lizard brains, sleep-related activity in deeper brain regions was disrupted. This confirmed that the reptilian cortex actively regulates sleep, just as the cortex does in mammals. The cortex and a deeper structure called the claustrum form a reciprocal circuit, bouncing signals back and forth in a way that mirrors the mammalian arrangement. Sleep, in other words, isn’t just passively happening in reptile brains. It’s being actively managed by some of the same organizational principles that govern your sleep.
How Old Is REM Sleep?
Reptiles and mammals last shared a common ancestor roughly 300 million years ago, during the Carboniferous period. Birds split from the reptile lineage later. If REM-like sleep exists in all three groups, one possibility is that it dates back to that ancient common ancestor, making it older than dinosaurs.
But the picture is more complicated. The differences between species, like the tegu’s unique 15-hertz oscillations and its lack of a regular sleep cycle, suggest that sleep states may have evolved independently more than once. Some researchers argue that the similarities point to a shared origin. Others believe the variations are better explained by convergent evolution, where different lineages arrived at similar solutions to the same biological problem. A 2022 review in the journal Biology concluded that sleep in non-mammalian species likely arose through “multiple processes of evolutionary convergence” rather than simple inheritance from one ancestor.
Do Reptiles Actually Experience Dreams?
This is where the science hits a wall. REM sleep in humans is strongly associated with dreaming, but REM sleep and dreaming aren’t the same thing. People sometimes dream during non-REM sleep, and not every REM period produces a dream report. Dreaming is a subjective experience, and there’s no way to ask a lizard what it perceived during the night.
What we can say is that the ingredients are present. Reptiles cycle through brain states that include the electrical signatures associated with dreaming in mammals. They show rapid eye movements. Their muscles relax. Their brain activity during these phases mimics wakefulness. If a sleeping reptile’s brain is doing something with all that activity, it may be processing experiences or replaying sensory information, functions closely tied to dreaming in mammals.
The honest answer is that reptiles have REM-like sleep, which is the closest measurable proxy for dreaming that neuroscience can offer. Whether a bearded dragon sees images behind its closed eyes or a tegu replays the day’s foraging route remains a question no electrode can yet answer. But the biological machinery is far more similar to ours than anyone expected before 2016, when the first recordings from sleeping dragon brains changed the field.