I am currently downloading a legal torrent of 49 files (8 GB total) and getting the speed of 50 KBps. How does doing this overnight affect the CPU and hard disk? Are there any long-term risks to the hard disk by the constant write or the CPU?
The CPU is a tough piece of silicon and due to the protection system built into recent CPUs, you can't really damages it by overheating.
There should be virtually no wear in the HDD once the head started flying. If during the night, the drive does not experience extra load/unload or spin-up/-down cycles, the extra hours of work should have little effect on its life. However spinning hard disks are sensible to heat and vibrations, if you have a poorly ventilated/dusty case, your drive may die an early death.
There is a study from Google: They found that load does not affect a hard-disk's lifetime in any way. It really does not matter if you torture it all day or not. http://static.googleusercontent.com/external_content/untrusted_dlcp/research.google.com/en//archive/disk_failures.pdf
The CPU though will have a shorter lifetime due to heat. Heat increases migration of particles. But downloading a file uses the CPU to a very tiny degree. Don't worry. I never do worry about stressing system components.
That covers the CPU, which as pointed out is one durable and amasing piece of a computer.
A hard drive allowed to sleep at night could give it 6-8 hours of bearings of the motor downtime. While it isnt great to have a hard drive spinning up and down all the time for no good reason, allowing it to spin down for half days, can extend the life of the motor bearings by that ammount of time.
If the hard drive is allowed to sleep,and would sleep, and downloading All Night long keeps it awake, then it will last less time. If it is a download or anything else "finishing up" then the drive can still go to sleep eventually.
when it comes to night vrses day , everything being a bit cooler is a good thing, and often the network is less utalised at night, so the activity is less likely to get in the way of other people in the local area.