Why getting ready for bed is thirsty work: Our internal body clock triggers a desire to drink as we feel sleepy

Most people will have woken at some point with their mouth feeling like it is full of dry cotton wool, but it seems our brains have a built in mechanism to prevent this.

Researchers have found that the brain's internal biological clock actually stimulates our thirst in the hours before we go to sleep.

The findings may help to explain why many people can feel the need to have a drink right before going to bed.

The researchers discovered a signalling molecule is released by the region of the brain that regulates the internal body clocks of mice to trigger a response in 'thirst neurons' as it starts to get dark.

Although the research was conducted in mice, it could also lead to new ways of treating people who suffer from jet lag and have trouble adjusting to shift work.

Professor Charles Bourque, a neurologist at McGill University, in Montreal, Canada, said: 'All our organs follow a circadian rhythm, which helps optimize 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.'

Circadian rhythms are the natural cycle that all the cells in our bodies undergo and are regulated by daylight.

A region of the brain called the suprachiasmatic nucleus (SCN) - a tiny region in the hypothalamus – is known to regulate the circadian rhythm.

The researchers, whose work is published in the journal Nature, found that a neuropeptide called vasopressin is released by the suprachiasmatic nucleus plays a critical role in triggering the sense of thirst before bedtime.

Rodents often show a surge in water intake during the last two hours before sleep.

Using cells designed to fluoresce in the presence of vasopressin, the researchers were able to show that it turns on the neurons that govern thirst in a 'hydration sensor' of the brain.

The study shows that the drinking behaviour in the mice is not motivated by an immediate physiological reason such as dehydration but may be preemptive.

The researchers also found that restricting access to water in the hours when their brains lit up with this activity resulted in significant dehydration towards the end of their sleep cycle.

It suggests that the increase in thirst is a mechanism to guard against this dehydration towards the end of their sleep cycle.

Professor Bourque said: '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.'