A reader asks:

 My husband and I practice natural family planning using the sympto-thermal method. Charting my waking temperature at the same time each day is important to identify the thermal shift that occurs after ovulation. I would like to wake up after taking my temperature, but never do. Right now, I don’t see a problem with applying CET’s timing prescription for optimum light therapy (Automated Morningness-Eveningess Questionnaire, AutoMEQ), in which case I would wake up and use the light at 7:45 AM. However, will using this therapy possibly artificially alter my normal waking temperature?


Good and important question. Morning light therapy is designed to shift your circadian rhythms earlier, which is a key factor for (a) the antidepressant effect, and (b) counteracting the pressure to oversleep. Body temperature — especially at night — is strongly influenced by the internal circadian clock. Therefore, you can expect that your morning temperature before getting out of bed will be higher after you begin light therapy. That is not because light therapy raises temperature, but rather because morning light therapy shifts your entire temperature rhythm earlier, by perhaps 1-3 hours. (Your lowest point on the 24-hour temperature cycle–the temperature “nadir”, which occurs about 2 hours before waking–will also shift 1-3 hours earlier.) Therefore, you can expect that your wake-up temperature will be higher all across the month. However, your daily record will quickly re-stabilize, and its shape across the month will be similar to what you see now.

All that said, the important news of 2006 is that morning light therapy also regularizes the time of ovulation. Ovulation thus becomes more predictable, and in women with cycles longer than 28 days, the cycle can normalize. Indeed, morning light therapy has a stimulating effect on sex hormones, and can serve therapeutically to increase the chance of ovulation at mid-cycle at the same time it normalizes cycle length. As such, morning light therapy can serve two important goals: guarding against pregnancy in women who want to avoid it, and enhancing the chances for women aiming to become pregnant.