For power control of your Mac from your iPhone on a local network, I have used Off Remote which can also schedule within the next 24 hours a system action (Shutdown, Lock, Restart, Log off, Sleep) and is only $3 in the iOS store but requires installation of an additional helper app on your Mac to receive the instructions from the iPhone app.
Another answer here suggested using Power Manager software which costs $50, but I would suggest the $36 Keyboard Maestro software which is a bit less intuitive than Power Manager but has greater flexibility beyond power management. In searching for the ability to schedule my Mac to sleep, I remembered that I have this utility and was able to quickly drag and drop the macro components in KM to create a script that paused for a set period of time and then put my Mac to sleep:
…there is also an action within Keyboard Maestro to also schedule macros (to put Mac to sleep, shutdown, restart, etc.) to be triggered at designated times:
Keyboard Maestro is cheaper and much more robust for developing macros that can enhance the use of your Mac, but there is a bit more of a learning curve than the Power Manager application.