Statins are drugs like atorvastatin  which inhibits cholesterol synthesis by inhibiting the enzyme HMG Co A reductase in the liver. The enzyme is more active at night. Drugs which inhibit it will have better effect when taken at night. This is more so with earlier drugs like lovastatin which were shorter acting than atorvastatin. So it is better to take statins at night. Since atorvastatin is a long acting drug, it will have its effect even if it is taken at other times of the day.