According to Dr. Vitiello, an internationally recognized expert in sleep and sleep disorders in aging, low levels of sodium in the blood cause blood volume to decrease, and the sympathetic nervous system becomes more active in order to compensate. That causes sleepers to wake up more often and have difficulty going back to sleep.
A research study looked at the sleep of nine 23 year old men who were placed on low, normal and high sodium diets.
When they were on low sodium diets, about 0.5 grams a day, the men woke up during the night nearly twice as often and got about 10 percent less sleep than when they were on normal diets, which included 2g daily.
When they were on a high sodium diet, 5g a day, the men slept longer and woke up less at night than when they were on normal diets.
If you have been on a self imposed restricted salt diet (do not go against doctor's orders)and want to experiment with increasing it slightly, unrefined sea salt is said to be a much better source of salt than the chemically derived table salt commonly used, as sea salt contains many other beneficial compounds including minute quantities of potassium and magnesium.
However, scientists have cautioned, that people with high blood pressure should continue to keep their sodium intake low, because the benefit from a low sodium diet with respect to blood pressure outweighs its detrimental effect on sleeping.