Simvastatin and other statin drugs lower blood cholesterol levels by blocking an enzyme called HMG Co-A reductase. This enzyme is made in the liver. It drives a key step in the making of cholesterol.
The liver makes more cholesterol at night. So you get a little more cholesterol-reducing activity from each pill by taking it at night. The medicine will be at its peak strength at the very time the liver is producing more cholesterol.
But plenty of cholesterol is made during the day. Many of my patients, for one reason or another, forget to take their statin pills in the evening too. I tell them that its okay to take them in the morning. Yes, simvastatin does work better at night. But you are better off taking it every morning rather than missing doses.