You can definitely set it up to be alerted when they are down... Page/Email, etc...
But a random email to say "everything is OK" is a rare request...
Unless someone else has a way they do this, I'd say configure your critical node down alerts to notify you immediately..
It should be possible.
Create a new advanced alert with the following condition:
Node Status is equal to up
If needed add more conditions to match
certain monitored nodes
In the Trigger Actions, click on Add New Action and select "Send an E-Mail/Page"
In the Time of the Day tab, select your shift hours.
Click on Alert Escalation tab and select "Execute this Action repeatedly while the Alert is triggered".
Set the "Execute this Action repeatedly every" to e.g. 3 hours.
The alert should trigger few seconds after you click on Ok and stay triggered all the time. If other alerts are already taking care of the Down status, no need to configure any Reset Action for this new alert.
That was my 2c .