The relations between insomnia and blood pressure were investigated in 151 patients (56 men and 95 women). It was found that 37.1% of the patients were normotensive, whereas 31.8% suffered from arterial hypertension and 31.1% from arterial hypotension. There was practically no difference between normotensives and hypertensives in respect of disturbed behavioral patterns of sleep, but an entirely different picture was presented by the arterial hypotensives. Characteristic features of hypotonic insomnias were (mostly in women) prolonged time before falling asleep, frequent awakening at night associated with tachycardia and long-lasting increased excitability, starting difficulties in the morning, depressive conditions, compulsive yawning and falling asleep during daytime, tiredness, lack of "drive" and reduced physical and mental efficiency. This pattern of signs and symptoms was enhanced by hypnotics and tranquilisers. During sleep blood pressures down to 50/35 Torr were measured. It is assumed that the low blood pressure results in hypoxic and hypoglycaemic conditions in the brain. Spontaneous awakening is considered to be an emergency reaction triggered by the deficient cerebral blood flow.