If the heat is keeping you awake at night, don't be surprised if the morning sun brings literal physical pain.
Scientists say just one sleepless night is enough to send the brain's pain-sensing regions into overdrive.
And to make matters worse, a lack of shut-eye also weakens the brain's ability to handle pain.
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Scientists at the University of California kept people up overnight and watched their brain patterns.
"When the researchers kept healthy young adults awake through the night in the lab, they observed increased activity in the primary somatosensory cortex and reduced activity in regions of the striatum and insula cortex during a pain sensitivity task," the Society for Neuroscience said in a statement.
The primary somatosensory cortex helps in processing pain signals, while the insula cortex figures out how bad the pain is.
And an online survey found volunteers reported increased pain the day after sleeping badly.
"These results suggest improving sleep quality, especially in hospital settings, could be an effective approach for pain management," the Society for Neuroscience said.
"More generally, the research highlights the interrelationship between sleep and pain, which is decreasing and increasing, respectively, in societies around the world."
The study was published Tuesday (NZ time) in the journal JNeurosci.