The changes which our body orchestrates that prevent dehydration at night, when we sleep are reported in a study published online this week in Nature Neuroscience.
The amount of water retained by the body is controlled by vasopressin, a hormone released by neurosecretory cells in a brain area known as the hypothalamus. These cells are activated by osmosensory cells, which track the concentration of water in blood. The hypothalamus also contains cells that form the suprachiasmatic nucleus, which is thought to contain a central body clock. Daily, rhythmic changes in the activity of the cells in this nucleus influences all of the body's daily rhythms, from when we feel hungry to when we feel sleepy.
Eric Trudel and Charles Bourque studied the connection between these three groups of cells. They found that late at night, the connection between the water-sensing osmosensory cells and the vasopressin-releasing neurosecretory cells grows stronger, so that even slight dehydration results in a greater release of vasopressin. Also at this time of the night, activity in the suprachiasmatic nucleus reduces. The team artificially increased the firing of the suprachiasmatic nucleus cells and found that the connection between the osmosensory and neurosecretory cells grew weaker.
These results suggest that that activation of the suprachiasmatic nucleus weakens the connection between osmosensory and neurosecretory cells, acting like a 'brake.' During the day, we can remedy low water concentrations merely by drinking some water. However, when we sleep during the night, the activity in the suprachiasmatic nucleus reduces, releasing the 'brake', and allowing more water retention.