The symptoms you are experiencing: fast heartbeat at night, especially when waking to use the toilet, along with sleep problems, are commonly seen with anxiety, light or disturbed sleep, or sudden changes in body position, and are often called nocturnal palpitations. These can sometimes be influenced by stress, lifestyle factors like caffeine or late meals, or even medicines such as Clonazepam, so it is important not to change your medications without medical advice. In most cases, this is not serious if the heartbeat settles quickly, but maintaining good sleep habits, avoiding stimulants, and practising relaxation techniques can help. However, if the palpitations become frequent, prolonged, or are associated with chest pain, dizziness, or breathlessness, you should consult your doctor for proper evaluation and review of your treatment.