This graph shows the action profile for Lantus (Glargine, the dotted line at the bottom):
As you can see, it peaks between 2 and 16 hours after it has been injected. There is not a lot of action during the last 8 hours of the 24 hour cycle. This is why it is a good idea to split the dose, and make the injections at least 8 hours apart.
As far as the the dosage amount is concerned, 20 units sounds like quite a lot. I weigh 73 Kg and use 15 units of Lantus a day, split into two shots (7 units before bed and 8 units in the morning). Some people inject the full dose before going to bed. The peak action counteracts the DP effect so morning numbers are good. And the reduced action during the rest of the day is adequate because of increased insulin sensitivity after lunch. This is a common pattern, but you will have to try out the different options and see what works best for you.
