Wife uninterested in sex. Lead in bed?

I am a 36 year old married male. I have ben married for 8 yrs. Of late, my wife has lost interest in sex. We don't have sex as often as we used to do about 3 yrs back. I do understand that our lifestyle has changed in last 3 yrs. She wakes up early in the morning, prepares breakfast and gets the kids ready for school. After that, both of us leave for work. At the end of the day, she is too tired to even think about having sex. She just goes off to bed after finishing all the household chores and falls asleep pretty soon. Is there any way I can convince her for sex at-least during the weekends ? Is there any medicine which I can give to her without her knowledge which will help her get into the mood for sex and take the lead in bed ?