To the Editor.— The letter of Major Allen B. King appearing inTheJournalJune 26, 1972, on the subject of hypoxia and hypercapnia 1 from bedcovers was of particular interest to me because it suggests the probable pathophysiology underlying a type of headache that I have studied in a number of patients. This headache may awaken the patient from sleep during the night, or is present upon first awakening in the morning. It is generalized with bifrontal preponderance, and is similar to the headache of chronic carbon monoxide poisoning. 2 Because of the timing of its occurrence almost every morning upon awakening or during the night, it frequently raises suspicion of a brain tumor. However, on questioning these patients, it is found that they have the habit, as described by Dr. King, of pulling bedcoverings over the head upon retiring. I have thus referred to the resulting headaches as, "turtle