For the third night in a row, the Humber Teaching NHS Foundation Trust confirmed that the unit would close at 8pm.
Confirmation of the temporary early closure for the rest of the week has been confirmed "based on the current staffing position."
A spokesperson from the Trust said: "We would like to reassure the public that the Trust is working hard to resolve the issue and we will resume normal opening hours as soon as possible.
"The decision to close at 8pm will be reviewed on a daily basis as the contingency arrangements are confirmed.
"The MIU department will have signage displayed to redirect visitors.
"If a visitor arrives with a medical emergency, the sign and buzzer next to the MIU front door should be pressed to directly call the Yorkshire Ambulance Service, who will assist from that point. "
"All other visitors will be instructed to call NHS 111 and seek advice from call handlers on alternative assistance."
The regular evening sessions, between 8-11pm, are "the least busy period for the MIU with usually less than 3 people per evening", they added.
The unit will reopen as normal at 8am during the week.