[skstrials]

I would like to use my Dell Venue 11 Pro Windows tablet for the morning alarm clock.

And I would like to know if there is any harm in leaving the tablet sleeping over night at all times.

If I want to hear the alarm clock in the morning, I at least have to leave the tablet sleeping instead of turning it off fully.

Cheers.

[xandros9]

There is no harm.
Only negative would be less battery in the morning.

However, I personally recommend the occasional restart.

[skstrials]

How often should I perform a restart if I just use sleep mode?

[xandros9]

How often should I perform a restart if I just use sleep mode?

Eh, I don't use any specific number. My aging laptop can act up after a day or two, but in your case, maybe every few days, until you notice odd behavior (like a glitch, it wont start talking) or just for new updates.
Use your discretion, you'll probably come to know its temperament well.