For maximum life of your pc I would suggest a couple of things. Firstly, the most fragile part of your system in regards to shutdown/hibernate/sleep will be your motherboard and components connected directly to it (PCIe cards). These components are receiving an electrical charge while your pc is running and when you shutdown the pc the charge stops, and returns when you boot up again. This can result in power fluctuations that actually damage or shorten the lifespan of your components, especially when power surges or spikes occur. Typically, and especially with newer 'green' hard drives that are not continuously spinning, hard drives can last an extremely long time with the pc power either running or in any mode. Remember, hibernate will simply copy your ram into your hard drive temporarily so you can resume your session later, however, essentially shuts down power to your components in the same way as a system shutdown. Sleep will keep your ram powered, but will shut down power to other components. In short if you are concerned about all components lifespan I would suggest keeping the power on, however, adjusting your power/performance settings to your liking in windows settings. If you are mostly concerned with just RAM lifespan, you can put the pc in sleep mode and have no worries.
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