When I start seeing a big pull downwards around the 5-8 hour mark after giving morning Lantus coupled with overnight numbers trending upwards then I suspect the Lantus might be getting old. I keep all new vials of Lantus in the fridge until I start using it. I then only use it for one month. As the Lantus gets older it is the property of long lasting that seems to fail. The insulin still works, it just starts working quicker and runs out sooner. If you have not started a new vial recently, then I would try that.