Tutorial about sleep modes: https://www.gammon.com.au/power.
The Arduino Mega 2560 is the same AVR family microcontrollers as the Arduino Uno. Most of the tutorial is for the Mega as well.
No, you can not clear the SRAM.
You can test that with unused SRAM. Store some data between the top of the heap and the bottom of the stack, press the reset button, and that data is still there.
The SRAM does not lose its data during a sleep-mode. All the variables keep their value. The clock signals uses energy, but keeping data in static ram does almost not use energy.
Some Arduino users jump to "vector 0" to restart an Arduino. That is not safe, since the microcontroller puts all the hardware in a off-state at powerup.
The WatchDog can be used to issue a real reset, just like pressing the reset button.
Can you tell more ? Why is the SRAM so important ? Do you want to clear it or do you not want to clear it ?
(maybe you noticed that I'm confused)