As the comments have mentioned, the purpose of the MOSFET is not clear. I can only assume you want to turn it on and off each time you want to measure the battery voltage. First of all, you want to use a PMOSFET for this, rather than NMOSFET.
But, after saying that, it is not even needed. All you need is the battery voltage and a divider. You can still take the battery measurements at whatever intervals you want in your code, you don't need a MOSFET to do this. If you make sure your resistor values are quite high, then you can minimise current.
You may also consider using a buffer as well to feed the microcontroller pin to give a more stable reading to the MCU AIN pin:
simulate this circuit – Schematic created using CircuitLab
Using a buffer will stabilise your reading, so it won't get loaded down by anything, and change the reading. This will allow you to use higher value resistors in the divider, to further save power.