If your contract stipulates that you will work 38 hours per week then your employer cannot, lawfully, unilaterally change the terms of your employment to reduce your working hours to 19 per week.  If your employer does change your terms in this way, without your consent, then you may have the right to resign and claim that you have been constructively dismissed.

Alternatively, your employer may dismiss you and offer re-engagement on the new terms.  If it does so, your employer must comply with the notice provisions of your contract.  It must also have a potentially fair reason for dismissing you, and follow a fair and appropriate procedure in reaching its decision; otherwise, if you have been employed for over a year, then the dismissal may be unfair.

Howsoever your employer makes any such change to your terms of employment, if such action is only taken in respect of you and this is because of your age, this may be directly discriminatory, unless your employer can justify the discrimination as a proportionate means of achieving a legitimate aim.  Whilst a legitimate aim could, potentially, include protecting your health, it is unlikely that protecting you from being overly tired would qualify.

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