I have never found that it runs out by the end of the day as the tablet is a slow release and stays in your system for 72 hours. If you are eating a lot of acidic food or acid making food in the evening or drinking acidic / fizzy drinks like wine, or pop, you may still get some reflux symptoms. It may be that you need to be on 40mg rather than 20mg, if you are not on it already. Also, if you are suffering from a bout of oesophagitis, it can take up to a month for that to settle down even with Nexium because the oesophagus is inflammed and sore and needs time to heal.
The best thing that I did to help my symptoms was to raise my bed head by 6 inches and use a wedge pillow on the matress as well. I got my wedge pillow from a company in America. It cost £100 to get it shipped to the uk but it was so worth it. The company is Medslant. I have a wonderful nights sleep with no acid burning/reflux in the night. It also reassures me that my oesophagus is not being damaged by acid in the night. The only down side is that I do slip down the bed by morning, but I can cope with that. Also allow 2 hours after the last food you eat before you go to bed.