I don't think there is legitimate study out there to back anything up since there a few variable factors there such as time most notably.
Having said that, here is my two cents, or my "study" that is worth two cents.
I think of computer as a car. When you start a car up, it takes more gas to start it up and some more time and gas (depending on how old the car is) to warm up the engine. If you were to start it up and not turn it off, considering you have a constant supply of fuel, the engine will wear out faster. This, of course, considering you do a routine fluids check and change and the typical maintenance.
The flaws here are: it isn't necessary to keep the car on all the time for time to time usage with downtime included. It also causes an OUTRAGEOUS gas bill and if you're the type that is for conservation, it is SATANIC VILLIAN for the environment. And also, the engine will unnecessary wear out for it was only genuinely used from time to time will being on idle standby all the time.
This translates over to the computer. It is true that when you start up your hard drive to boot, it does put more wear on the drive than in use. But that is limited to a few seconds or so and then levels out during normal use of your computer. If you were to keep the hard drive on all the time, it will wear out faster because of heat build up in the disk, the actuator, the actuator arm, the spindle and such. Unless if you keep it really cool and the airflow very cool and clean, the drive will fail resulting in replacement costs, the time needed to reinstall Windows and the time needed to reinstall and reconfigure your machine and settings. That is what will happen to the hard drive.
The other components would be able to stand up to being on all the time, but then, the RAM will need a refresh due to some error or such. That is though considering, the airflow will need to be cool and clean to prevent dust build up.
That may not go so well for the power supply though. The other parts, excluding the hard drive, are able to be powered down from software setting so they don't consume so much power, which reduces heat, which reduces wear on the parts. The power supply has to regulate the power flow and do the AC to DC power conversions. That in itself will produce heat energy. It's the main source point all power in a computer. Without clean and cool air by default, there is a higher chance the power supply could fail as well because it has a HUGE stream of electron flow, and a HUGE stream of electron flow results in heat in the wiring and heat isn't a good thing anywhere.
To conclude my thoughts and ideas on this two cent study, you should at least put your machine to sleep. If you don't want to wait for Windows to boot, just put into sleep mode, or even hibernate and restart from time to time to clear out the RAM. The wear on the hard drive during a reboot is rather negligent towards the life of the drive, and other parts as well. It also will save you money and time having to replace the hard drive or other parts for that matter, as well as money on electricity.
Now for the real part of the study.
I happen to been through the use of a machine that lasted for about eight, nine years. It was born of xp and died with 7...it was a national tragedy.... It was constantly on every week and was shut down every day or few days depending if there was torrent downloading being done. Two years ago, the hard drive finally died after the motherboard was replaced due to some power fault I believe. Hard drives have a data retention span of about ten years if I remember correctly and limited power on hours, so that hard drive lasted close to what it should have been at. And keep in mind that hard drive was built in 2001 technology.
The motherboard died from some power issue, I think something shorted out. The power supply is still fine, I actually still have it.
The moral of this story is, there is reason to shut down or sleep for that matter. If you mean the computer itself, as in the processing components, will last being on for so long; those parts will remain working if they're on all the time. But if you mean the hard drive, it will not last that long compared to being shut down or off nightly. This is all.