I am 74 years old and have been taking 50 mg dose of Amitriptylin...

I am 74 years old and have been taking 50 mg dose of Amitriptyline for 12 years and I take it two hours before bedtime. Recently it has been causing sleep apnea. I am now taking 25 mgs, two hours before bedtime at 8pm and need to know what time should I take the second dose in the am? I took it this morning around 9am and was sleepy most of the morning. I am guessing this has to do with how long between the dose the night before and the am dose was taking. How long between these 25 mg doses should I take them. Thanks Much. D.