Personally, I think there is no best time to send emails as a general rule.
However, for you to know the best time to send emails to your list, you need to test.
This might be possible after 3-4 weeks of consistent (daily or twice weekly) sending of emails to them.
The location of your audience is also a contributing factor.
If you are building a list for audiences not restricted to one location in the world, you should know that they will receive your emails at different times.
There are some Email Service Providers that allow you send emails to your list at the best time they are more likely to open their emails. I have seen this in Getresponse and it's a good way to help you get more open rates for your emails.
So, there's no general rule that your emails should be sent at a particular time.
No, there is none that I personally know of.
Find out what works for you.
That's it for today.
Talk to you soon.