Hi there,
Zopiclone is a CNS depressant (along with opioids, alcohol and benzos) and so if combined with an opioid there is a risk of respiratory depression which can be fatal - combining CNS depressants often has a synergistic effect, so the overall effects are greater than the sum of the two parts, and combinations are a very common cause of overdose. The zopiclone is obviously adding to your sedation and increasing your nod, which is often the goal of opioid users, and a nod itself isn't dangerous as long as you are breathing okay - but if you feel your breathing slowing down or if you feel over-sedated then please seek help immediately. While nodding itself isn't dangerous, it is about as sedated as you can be on opioids without straying into danger territory, and there is always the risk that you might tip yourself over - people can nod first then slip into respiratory depression/arrest/unconsciousness/death so it is really important to know your own limits and be careful. You're taking high doses of both drugs; what is your tolerance like? Do you really need to take so much? If it is worrying you then I strongly suggest reducing your dose.
If you are ever concerned for your safety, please seek medical help straight away. We can't tell you if you are okay or not over the net.. at the very least please get someone to sit with you next time (it doesn't sound like you are on this combo at the mo?)
Also, you shouldn't take more than 400mg tramadol/24 hrs as there is a risk of seizures. The recommended daily max is 400mg; people have seized on less than this and the risk is a very real one. People vary in their seizure threshold, but just because you've been okay with 600mg in the past doesn't mean you necessarily will be in the future.
28mg is a lot of zopiclone too..