Have You Ever Wondered Why You’re Not Thirsty At Night?

Published On March 2, 2010 | By Tomás Bosque | Articles, dehydration

I love water. I also enjoy sleeping. But I’ve always wondered why I don’t usually need water during the seven hours I sleep at night.

A recent article I read noted the physics behind why we don’t get too dehydrated at night and I’ve posted it for your viewing pleasure below.

Brain cells collude to keep animals hydrated while they sleep, which prevents nightly dehydration or trips to the toilet, according to researchers.

Neurophysiologists Eric Trudel and Charles Bourque at the Research Institute of the McGill University Health Centre in Montreal, Canada, suggest the body’s internal clock helps to regulate a water-storing hormone.

They insist the mechanism regulates the body’s circadian system, or internal clock and controls water regulation.

The body regulates its water content mainly by balancing water intake through thirst with water loss through urine production.

Scientists had the knowledge that low water levels excite a group of cells called osmosensory neurons, which direct another set of neurons to release vasopressin, a hormone that instructs the body to store water, into the bloodstream.

Vasopressin levels increase during sleep; clock neurons, meanwhile, get quieter.

Trudel and Bourque wanted to test the idea that lower clock-neuron activity might allow osmosensory neurons to more easily activate vasopressin-releasing neurons, which would mean more water retention and less urine production during sleep.

For this, they isolated thin slices of rat brain containing intact sensory, vasopressin-releasing and clock neurons. Even when removed from the brain, clock neurons continue to mark time.

The pair then stimulated the sensory neurons and recorded any electrical activity in the vasopressin-releasing neurons to monitor communication between the two cell groups.

They then moved on to look at the effect of the clock cells on this pathway. When they did not activate the clock cells during the ‘sleep’ part of their cycle, it was easier for the sensory ells to communicate with vasopressin-releasing cells.

Conversely, when they activated the clock cells, this communication decreased markedly.

The results indicate that clock cells function as a dimmer switch for water control. When their activity is high, they prevent sensory cells from instructing secretory cells to release vasopressin. Then, when clock cells are less active, sensory cells can easily instruct secretory cells to release vasopressin, ensuring that the body holds on to its water reserves.

Colwell points out that despites rats, are nocturnal, the vasopressin cycle and clock-neuron activity are similar in rats and humans.

“We show this for this one circuit, but it’s possible that clock neurons regulate other circuits in a similar manner and this remains to be studied,” Nature quoted Bourque, as saying.


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