According to the symptoms you report (“dozing off”, “being suddenly jolted awake with a racing heart”, “fear of trying to go to sleep”…) and the normal ECG and holter monitor, you might be experiencing some kind of somatoform disorder. Somatoform disorder is also called neuron-vegetative dystonia. It is a kind of neurotic disease (classified as F45) wherein psycho-emotional problems manifest as symptoms of other organic diseases. For example, chest pain can be a symptom of angina pectoris; a patient experiencing somatoform disorder can also feel this symptom without being diagnosed with angina pectoris. To establish the diagnosis for somatoform disorder it is very important to first exclude any real organic disease. In your case, heart arrhythmia had to be excluded with ECG and holter monitoring. Panic attacks (F41.0) are another possible diagnosis, but it is not typical for panic attacks to occur during the sleep. You may consult a psychiatrist for further information.