A reader asks:

I used to be a night owl only — I could not fall asleep until 1-2 a.m. and then always had trouble getting up . . . until I had radiation treatment for cancer! After that, I became a complete morning person, waking at 5 a.m. every day and falling asleep by 9 p.m. every night. Now I have zero energy in the evenings after 7 p.m. Could the therapy have reset my clock? Could I use light therapy to readjust later?


Yes, and yes. Since the internal biological clock, which lies at the base of the brain, is a nucleus of oscillation neurons, it is possible that radiation altered its timing properties irreversibly. That does not mean your extreme morningness is irreversible, however. You could try light therapy — for starters, around 7 p.m., when the evening slump sets in. You may find that it gives a significant boost, which could allow you to resume a later, “normal” bedtime.