If you read the specifications for mechanical devices in your computer, for example, disks, fans, and DVD readers, or even pushbuttons, they all have a rating for number of actions.
Disks are guaranteed for aseveral hundreds of thousands of start stops, The same is true for mechanical switches (ons and offs), such as power, reset, keyboard, mouse buttons, wheel, etc.).
If you look at your desktop computer, you will probably note the lettering or ornamental plating on the front power switch is worn away. So, big deal.
If you have a laptop, it is best to power the unit off. This will tend to preserve the battery. But if you want faster startup, use the sleep mode. Sleep mode is best only if you are going away for an long bathroom break and you want privacy, and will be back to full power rapidly to continue using the system from where you left off.
Hibernate is a concept that says, "Write a full image of memory to a cache / swap file and then power off."
Startup should be somewhat faster than going through a full boot. The negative side is that any system updates for security, bugs, etc, will not always be installed until a cold restart is done.
The full power off has a benefit of allowing a clean start the next time you begin to use it. Let me explain the benefits.
Not all programs are perfect with memory management. This means that after many hours of use, memory gets somewhat fragmented. Similarly cache/swap also gets fragmented. Some programs crash, and dynamic memory (some of the spare memory that was reserved), may not be given back to the operating system. Gradually, your system may begin to run more slowly.
So, in my view, for best execution performance, poweroff is best.
Some other ideas... If you have a desktop, check the filters that are usually on the case. Clean them of lint.
If you are handy, open the case, get a can of compressed air that is used for blowing ling away and do some cleaning, by blowing out the lint that is on the cpu fan and fins. Don't unplug or remove fans from aruond the CPUs. Blow or wipe away as much dust as possible, including the power supply fan. This action will do more to prolong computer life than anything else.
Regarding disks and disk spinning. All operating systems have an optional timed disk spin down. If you don't do any disk I/O for some period of time (a user setting), then at the end of that timeout period, the disk will spin down to a stop. So whether you choose to power off, put the computer in sleep mode, (power supply is running and so is the power supply fan), or put the computer in hibernate mode, the difference in life span after 5 years may only be measured in minutes.
Keep the cooling fans and cooling fins dust and lint free will do more than anything else to prolong computer life.