If you do not depend on a data connection, there may be some solutions around: The Google Playstore has a lot of automation apps to offer, which you can use to set up more than this. In your case, this would be a time-based scenario:
From 11pm to 7am, disable WiFi and mobile data
No data connection => no new mail => no new mail notifications. Apps capable of things like that include e.g. Timeriffic (free, and one of the top-apps in this section), Llama (also capable of location-based and other rules), and -- my favorite -- Tasker. Though the latter is not the cheapest one, it is the most flexible, and you can get a 7-day-trial at their homepage.
For examples what else you can achieve with Tasker, you might also want to take a look at What is Cell standby and how can I keep it from eating my battery?, and the examples at the mentioned homepage. Also, a Google search on Tasker profiles will list you a lot of goodies.
To accomplish your goal using Tasker, simply create the following profile:
- Time (fill in your "from" and "to" to define the time frame)
- Net->Mobile Data: Set Off
- Net->WiFi: Set Off
That's all. Say you defined it as described above (11pm to 7am), Tasker would switch off WiFi and Mobile Data at 11pm, and return to the previous state at 7am.
More solutions can probably be found here: Is it possible to only silence certain notifications?