Canadian researchers have found evidence that the body's biological clock could be why we get thirsty right before bed, possibly by triggering 'thirst neurons' to pre-emptively keep us hydrated throughout the night.
The team says a better understanding of the process behind this sleepy-time thirst could give us insight into other odd – and annoying – aspects of daily life that influence our circadian rhythms, such as jet lag or changing shifts at work.
"Although this study was performed in rodents, it points toward an explanation as to why we often experience thirst and ingest liquids such as water or milk before bedtime," said senior author Charles Bourque, from McGill University.
The team knew from previous research that mice increase their water intake before they sleep, but – since the mice were not dehydrated at the time they started drinking – the reason why was unclear.
To get to the bottom of what was going on, they first restricted the mice's access to water before sleep, which made them much more dehydrated when they woke up hours later.
This, the researchers say, means that the pre-sleep water intake is largely there as a pre-emptive measure to keep the mice hydrated all the way through their sleep cycle, which suggests that a biological mechanism was triggering the thirst, even though the body itself wasn't dehydrated.
While the team knew that the brain can sense when the body is in need of water, they hypothesised that the suprachiasmatic nucleus (SCN) – the region of the brain responsible for circadian rhythm – might also have a way to communicate with the 'thirst neurons' that trigger the mice to seek out water.
They tested this by specifically looking for the neuropeptide vasopressin, which is produced in the SCN and controls things like water retention and blood restriction, by using 'sniffer cells'.
These sniffer cells light up when they come in contact with specific things. For their study, the team injected sniffer cells specifically designed to light up when they came in contact with vasopressin into the mice, and then stimulated their SCN.
As soon as the SCN was triggered, the sniffer cells lit up, verifying that vasopressin is released in high quantities when the body's biological clock is electrically stimulated.
"We saw a big increase in the output of the sniffer cells, indicating that vasopressin is being released in that area as a result of stimulating the clock," Bourque said.
Finally, to see if vasopressin can trigger thirst neurons, they used genetically modified mice to turn neurons on and off, allowing them to observe what the neuropeptide does when released by the SCN.
In the end, they saw that vasopressin can turn on these thirst neurons, causing the mice to drink water despite not actually needing to.
This means that a mouse's biological clock - the SCN - has a built-in feature that can predict when sleep will start, triggering the brain to seek out the required resources - like water - to stay healthy during long periods of rest.
While it's important to point out that this research was done on mice, which means that we have to take it as a grain of salt until human trials are performed and verified, the team says that understanding this biological mechanism could help us to figure out many aspects of our bodies and how our daily lives can affect them.
"More importantly, this advance in our understanding of how the clock executes a circadian rhythm has applications in situations such as jet lag and shift work," Bourque said.
"All our organs follow a circadian rhythm, which helps optimise how they function. Shift work forces people out of their natural rhythms, which can have repercussions on health. Knowing how the clock works gives us more potential to actually do something about it."
The team's work was published in Nature.