My understanding, from experience is that reasonable adjustments apply if they can be put in place. eg if they can demonstrate that it's hard or much more expensive to find people who can cover your nights. On the other hand if they can easily cover your nights but refuse they are being unreasonable.
I worked in situation where it wouldn't have been possible to make such adjustments permanently, covering all night shifts (short staffed, hard to recruit skilled night staff, locum too expensive), but also in different places where it was achievable.
If you can't fulfil a contract, and reasonable adjustments are not possible, you can legally be dismissed.
I hope this information is useful, and good luck!