I first looked in the Body Clock Guide which has a whole chapter dedicated to the best times to make love. One study in it looked at the love lives of 5 Parisian men in the 1970's . They examined the levels of testosterone and other hormones as well as activity for 14 months.
During October there was a rise in testosterone corresponding to love making activity, which then dipped over 6 months. So maybe, Valentine's Day is just what we need in February!
|A young couple in love in Paris (C) Simon Moore, 2007|
Many of the other studies looking at the best time of day to make love but on the whole have been limited by small number of participants and accuracy. There's not been many recent studies, so I presume it's quite hard to get scientific funding for these trials. However, the results tend to show peak times of day are when it's most convenient, between 10pm-1am being most popular.
Perhaps more accurate data will come out of apps like Mappiness, where you can state making love as an activity option when it beeps you to find out where and how happy you are.
Happy Valentine's Day!
 Reinberg, 1978, "Circadian and circannual rhythms in sexual activity and plasma hormones (FSH, LH, Testosterone) of five human males" Archives of Sexual Behavior Vol 7, No. 1, pg 13-30, DOI: 10.1007/BF01541895