Running AT90USB646/Atmega32u4 between 3.3V and 5V
Since  power  increases with Vcc(I = V^2f; where I is current and f is frequency), you can reduce power consumption without limiting the maximum operating frequency by minimizing  the voltage of the device.
For example, running at 4.5V instead of 5.0V for a 16MHz device, drops power by almost 20% without compromising the device’s performance. As evidenced by the power  equation, reducing clock frequency also plays an important role in power savings. If you can optimize your code to run at 10MHz instead of 16MHz, current consumption is reduced by 30%-40%.
See Info here