Due to coming together of what does the dream arise?

Dreams are fragmentation/de-fragmentation of your memories.

Memories are based on a combination of your worldly senses from eyes , ears , nose ,tongue, body , mind....etc.

I guess there are two ways to explain it.

One is to say that it's the working of sense-consciousness without contact. When there's contact (with a sense-object) the contact combines with sense-consciousness -- sense-consciousness is conditioned by previous experience -- sense-consciousness combines with sense-contact to create perception ... for example, first you see something, and then you perceive or recognise it when you combine the sight with your past experience, and so can say "that's a dog" or "that's a road" etc.

I think the same process (i.e. experiences cause consciousness cause perceptions) sometimes continues during sleep, even without contact (I say "sometimes" because not all sleep is dreaming-sleep). I guess neurologists see activity in parts of the brain responsible for sight and/or perception, for example:

In addition the team found that dreaming about faces was linked to increased high-frequency activity in the region of the brain involved in face recognition, with dreams involving spatial perception, movement and thinking similarly linked to regions of the brain that handle such tasks when awake.

I guess that an alternative way to explain it is that dreams are mind-objects, involving contact between the dream and the mind-consciousness.

For example, even when you're "awake" you can remember things: e.g. remember the sight of a place or of a face, or the sound of some music or of a dog barking. When you do that (remember a sense-experience) I think you replay elements of that experience in some parts of your mind (e.g. in the parts of the mind which are responsible for perceiving or being perceived as that experience).

So if the first explanation above (sense experience and perception without contact) is difficult to accept (because it doesn't fit the doctrine of the 12 nidanas), I think the definition of the sixth sense (e.g. contact with "mind-objects") is broad enough to include the phenomenon of dreaming.

As for how it can happen without contact, I imagine a radio receiver. Perhaps you can't hear it properly -- when you can't hear it, you might turn up the volume (i.e. increase the amplifier). If you do that when it's not tuned to the signal from a transmitter, what you'll hear is "noise" -- that "noise" is the relatively small electric fluctuations which are present in the local environment (e.g. the radio's antenna and amplifier circuitry) -- normally the fluctuations induced by an external signal is louder than the noise, so what you normally get is contact with the signal, but if there's no signal you experience contact with the noise.

According to my analogy, dreams are "noise" that you experience when the louder signal (from the waking senses) are cut off.

One of the questions when meditating is whether to do that with eyes open or closed -- see e.g. What are the advantages/disadvantages of meditating with eyes closed?

The way I was instructed on Buddhism is very different from the perspective implied in most of the answers and in the question itself. My teacher(s) would probably say this misunderstanding comes from a deeply ingrained materialistic view.

From (right) Buddhist perspective, the five senses, plus the sense of introspection - are all mind faculties. They are differentiated by six modalities, but they are all faculties of the mind. To say that the world exists "outside" and the mind sits "inside" and perceives the world through the little holes is a (wrong) materialistic perspective. The world exists inside. The mind exists outside. There is no inside and outside.

When we're dreaming, we experience "a world" on the same five modalities, and we experience "self" on a sense of introspection - but this world and this self is not the same we experience when awake. Why? Because our state of mind is different. State of mind defines what we experience and how we experience it.

When Buddha talks about "contact" - he does not talk about an external object touching a sense organ. First of all, with the exception of taste, objects almost never actually "touch" our sense organs. The stimuli we do get are mere reflections of object's properties in either light-waves or sound-waves or its emission of chemical molecules (in case of smell). Even the sense of touch is usually mediated. When we feel the ground with our feet through the soles of our shoes, or even better - when we feel the surface of the road with the wheels of our car - the object never touches our body and yet we feel it. So the stimulus is not the object, the object is a construct of our mind based on the stimulus.

When Buddha talks about "contact", he talks about contact in our experience. Our constructed idea of self comes in touch with our constructed idea of object. This is called "contact". It happens in our imagination, whether we're dreaming or awake. In all cases the object is a projection of our mind.

-- "My question is: due to coming together of which internal and external does the dream arise? Due to which contact in this real world?

This idea of "external" and "internal" with the doors of senses in-between is a very materialistic idea. Also, there is no "real world", it's an experience, projection, imagination. My Zen Master called it "hallucination"!

As all experience (vijnana), dreaming arises from samskara. Then, "inside" that experience, there is "internal" and "external", "contact", "objects", "self" and "the world". All experience is constructed by the mind, including eye-experience, ear-experience etc. - both when dreaming and when awake.

A dream consciousness is a mental consciousness arising in dependence upon a mental sense power. While an eye-consciousness apprehends shape and color (which is form), a dream consciousness apprehends a phenomena source. A dream consciousness and its objects simultaneously arise from a seed left in one's mental continuum.

In his commentary to Lorig, Geshe Jamphel Gyaltsen said:

The blue in the dream is form included in the category of 'phenomena source'. We have the form source, related to the eye consciousness, etc. The phenomena source is the source for mental consciousness only. The blue in the dream is included in that category.

The only source taken in the sleeping state is the phenomena source and therefore more subtle. Between dream and deep sleep, in the dream you still have many different manifestations of the phenomena source, whereas in deep sleep you don't have these manifestation.

Not all mental consciousnesses have a phenomena source as their object of observation. For instance, the divine ear is a clairvoyance that is a mental consciousness apprehending sound, which is form.

Furthermore, in his commentary to Purbuchok’s 'Explanation of the Presentation of Objects and Object-Possessors as well as Awarenesses and Knowers', Geshe Tenzin Tenphel says:

The first, [a mental non-conceptual wrong consciousness] is, for example, a dream consciousness that clearly sees as blue the blue of a dream. This subject is a mental consciousness, a non-conceptual consciousness, and a wrong consciousness.

• it is a wrong consciousness because of being a consciousness that apprehends its object, a form which is a phenomenon source, as blue, whereas it does not exist as blue.

In other words, dream-blue is not actual blue. It is not form but phenomena source. Thus, 'seeing blue' in a dream is the function of a mental consciousness and one is not actually seeing.