Rozia - I work in the NHS so hopefully I might be able to offer a steer from inside the organisation, not sure whether anyone else who has replied, actually does.
Firstly, you have a contract of employment. It may not state your actual working hours but it WILL state how many hours per week you work. There is therefore no option for you "to involve HR" - they MUST be involved, because your manager will have to submit change forms to workforce altering your hours of work, and any workforce planning must show the rationale for making these changes. Your department's HR manager needs to know about this.
Secondly, your NHS employer will have an Equality and Diversity policy, in which it not only undertakes not to discriminate against patients because of their ethnic or religious backgrounds, but this also applies to the staff too - so find that on your intranet, have a read, again HR will be the best place to advise.
However - in all this, please do bear in mind that in our places of work, the patient comes first, and we are there to provide a service, and in many many cases at the moment, roles and hours ARE being changed across the country as every department is required to save money and to show how it's happening. If the changes in hours make your department more cost-efficient, that has to be their driver - unfortunately it does mean that it's not always so convenient for us as individuals. However - I'm not saying this means that you don't have some say in it - for any change of contract terms and conditions there has to be a consultation period with the employee (for us it's twelve weeks) so that the employee has the chance to put their side of it and have their say. It may not make a difference in the long run - but it has to be done, so that all views are known and explored.
So - step 1 - get in touch tomorrow with your HR manager - they can talk you through what's happening - and why - and tell you where you stand. (Please bear in the mind that the way it works with my NHS employer may not be exactly the same as in yours, but there are national guidelines and imperatives so IMO I wouldn't expect it to be that different!)