Science can finally explain why we get sleepy when we're bored.
The same 'feel-good' region of the brain that kicks in to help us enjoy food and sex is also involved in sending us to sleep.
Scientists from the University of Tsukuba in Japan found that this part of the brain is densely populated by receptors for one a molecule that makes us sleepy.
Whether you have had a full night's rest or none at all, the pleasure center of the brain will still make you sleepy if you're bored.
Boredom-induced sleep does not appear to be any different from regular sleep, and both can be fought off with caffeine, according to the study.
Japanese researchers found that the nucleus accumbens in the forebrain had an extremely strong ability to induce sleep.
The nucleus accumbens plays a key role in the brain's reward system, which releases dopamine and makes us feel pleasure when we do things that are productive for our survival, like eating, hydrating, having sex or even socializing.
But in the absence of these motivational stimuli, the same part of the brain appears to make us very tired.
We feel tired in accordance with our biological clocks, but 'sleep/wake behavior...is also influenced by cognitive and emotional factors,' the study authors wrote.
The nucleus accumbens has many receptors that respond to a neurotransmitter, called adenosine, which helps to regulate our sleeping and waking cycle.
Lead study author Yo Oishi, from the University of Tsukuba, said: 'The classic somnogen adenosine is a strong candidate for evoking the sleep effect in the nucleus accumbens.'
Scientists proved this phenomenon by using drugs and light to stimulate brain cells in the nucleus accumbens of mice, which, they found, caused the mice to sleep.
According to this study, published in Nature Communications, getting enough sleep does little to prevent you from craving a nap when you're bored.
The researchers found that sleep deprivation did not affect the way the mice's brains responded to a simulation of boredom.
But grabbing a cup of coffee or tea might help.
Caffeine and adenosine are part of the same chemical family. Caffeine essentially tricks adenosine receptors.
So, once caffeine molecules bind with those receptors, they won't respond to the adenosine's sleepy signals.
This discovery could help lead to safer treatments for insomnia as well, now that we know more about the location workings of these receptors in the nucleus accumbens.