I worked in healthcare I.T. for a very long time. You're 100% correct about time for recovery, that's very common.

Biggest reason though - generally the "A" team is on the day shift, you don't need your best, nor as many of the top talent on at night - those guys are on call but don't usually work at night. Scheduling just after the top-level I.T. staff's usual business day has become S.O.P in healthcare, and it means you have your A-Team on the maintenance task, and yes, usually you keep most all of that team on after to complete the task and manage any recovery, if required.

A maintenance task of an EHR system today may mean you need the Systems Engineers/Admins on hand who manage the actual EHR Servers, the Vmware team, the SAN guys, the network guys - in case you discover something is hammering the network, ports need to be changed to talk to the EHR provider - and you'll probably have some of the folks who coordinate/liaise between Nursing/Physicians and I.T... You'll probably even keep some software trainers around, and PC techs - in case there is a change that starts to confuse end users - or maybe a change to the software requires a change on the PC, and the PC techs can manually hit a few workstations as you're waiting for the changes to propotage though your systems management backend down to all of the workstations.

Properly done the front line folks will have no clue I.T. has a fucking herd of I.T. pros down in the basement and at the N.O.C., all at the ready to remedy any potential issues - and usually there aren't issues. But you keep them after hours in case you need them. Sure you can take your 8-5 staff and work them over-night, but that's becoming WAY less common, because the human brain doesn't work as well when a person hasn't slept.