Bern is a small city but a popular one and hotels get booked out very VERY quickly. It is highly advisable to book your accommodation as soon as you can. The following are a selection of hotels that are well-placed for the historic centre and public transport to the conference venue. 

The links take you to a Booking.com page for ease - and for photos and reviews. You can book direct with the hotel or via a hotel booking agency or use the online booking facility with Bern tourism (though here none of those all-important reviews...)

Travel passes: most ( all?) hotels give their guests a travel pass for the bus/tram/city trains for the duration of their stay.

DISCLAIMER:  EALTHY can suggest but cannot endorse, the hotels above.