As I understand it, once you take Januvia for awhile, you always have some in circulation (like the TZDs) - so you need to take it every 24 hours. The half-life is about 12 hours, so it peaks around then. I take it in the afternoon/evening to help keep my Dawn Phenomenon under control. That's why I take so much metformin at night too. My numbers are great during the day- 80's most of the time, but rise during the night no matter how low I am before bed, what time I eat dinner, what I eat for dinner, what kind of snack I eat or don't eat, exerceise before dinner, before bed, both - I've tested a lot of variables. My liver is just out to defeat me- so I need to beat it back. I take the second metformin with my largest meal of the day either lunch or dinner- it has a 2-3 hour half life - so hopefully is helping me to use all the glucose from my carbiest meal.
Januvia has stabilized my numbers, so that I don't have radical swings (like I did with glipizide), and I don't have to eat carbs to prevent dropping into the low 60's. It's there when I need it. It helped lower my AM numbers so that I am below 100 now 99% of the time (mostly mid to low 90's) as long as I take it between 4 and 8 PM ( I do take it later if I'm traveling and it's more convenient to do so)- no food needed with it, unlike metformin.
When my doctor prescribed it, she told me to take it when I would remember best- but close to the same time every day. When I travel, I try to take it once avery 24 hours no matter what the clock says (I travel from Eastern to Pacific time for work a lot, and to London 2-3 times a year.
I just want to live happily ever after-every now and then. Jimmy Buffett