Some problems are not important at night
As you suggest, Nagios lets you configure time periods which you can then apply to the checks, the notifications, the notifcation groups and individual users. Of course if you specify that the checks are to take place but no notification sent, then you won't see the problem in your email but you can look at the complete alert history at any time in the Nagios web pages. Why not schedule a cron job to email you the overnight alerts first thing in the morning? e.g.
0 8 * * * wget -O /tmp/nagios.html -i /home/user/nagioslist; mail firstname.lastname@example.org </tmp/nagios.html
Where /home/user/nagioslist contains something like:
(this would display up to 1000 hard states detected in the last24hours)
OTOH you could set up a different notification script to run for the notifications you want to defer and simply place them in a queue to be sent later (e.g. using 'at').
Or you could set up the MTA to process its outgoing mailq only at specific intervals - combined with different email addresses / different subjects then you can effectively prioritise/defer mail based on the recipient / subject / time of day.