No-brainer that IRCTC won’t allow you to book tickets from 11:30 in the night to 5 AM.
While there could be several reasons (batch processing?), the largest ecommerce site in India will extend its’ online booking service for 23 hours (timelines has not been announced yet).
The fact that e-ticketing is a hit among travellers is a matter that cannot be overlooked since a major chunk of the reserved ticketing is done online. NRIs and tourists from abroad have been complaining about difficulty in booking tickets due to the service not being available between 11.30 pm and 5am. The only time it will be inaccessible will be between 11.30 pm and 12.30 am for the date change- source/via
IRCTC recently started selling air tickets and is also the official partner for Common Wealth Games, 2010.