for a 6am flight, I've entered airside 11pm the night before
explored the airport (the game section is closed during overnight hours, butterflies are sleeping etc), a lot of stores are closed
even if you have bags, your airline may allow early bag checkin (up to 24-48 hours).
for SQ, it explicitly notes that you can only drop bags/enter airside 24 hours before flight, so 48-hour is a misnomer
for UA, no early checkin, though you should be able to get mobile boarding pass? this page from straitstime in 2016 doesnt mention United having mobile option at changi
https://www.straitstimes.com/singapo...rport-security
Departure, GST Refunds, Security - Singapore Changi Airport
SQ: Please note that baggage check-in and entry into the transit area are only available 24 hrs before departure.
UA: no early checkin available