Sharke, I've done some music a bit like you're describing although I'd categorize mine as "space" music or planetarium music.

My approach is (yes) lay down a really really long note or chord  in PRV and then audition a lot of presets in the Omnisphere playable textures category, find one that works and maybe apply a slowly evolving filter.

Then repeat the above a couple of times with different synths - I like Arturia CS-80 and Alchemy - and then add something rhythmic. It doesn't need a melody line but it should have something to convey an image or feeling. A good example is Steve Roach's music which is very good at transporting you to the middle of a desert.

Only guessing but I would think the ambient or relaxation type of music you mentioned would be done about the same. Heavily reverb ed flutes and whale calls and such put me to sleep tho.

Thing is, with "good" ambient music I just know I wouldn't be able to get to sleep because my ear's interest is piqued and wants to delve deeper into it. Like the Eno album Jeff linked to above. I just have to listen to five minutes of that to realize that it's got interesting stuff going on that my ear would want to follow if I was trying to drop off. 
 
I guess the music in the playlist which I fell asleep to last night was perhaps intended as sleeping music, in a similar vein to those white noise machines. I was just interested to know what the whole process was like when making music like that. 
 
Tbh I think whale calls would keep me awake btw! Or maybe I should try it. 
 
Here's an interesting one - the ambient sounds of a coffee shop to help productivity. I keep meaning to try this when I'm working because the times I've taken my laptop into these places, I do tend to get super focused. 
 
https://coffitivity.com/

With ambient music it also depends on how you listen to it as well. For example with Eno Ambient 4 if you parked yourself between your near field monitors and turned it up then yes you will hear all the interest and the detail much more.
 
But as Eno suggests too if say you are falling asleep and you have that music playing from a speaker set across the other side of the room and you set the volume lower then it all just meshes together and permeates across the room toward you and it becomes a more distant thing. All the detail rolls into a combined sound.
 
Hers is link to a folder on my Google drive with three Ambient tracks in it:
 
https://drive.google.com/drive/folders/0B_2Jb7O_b5BkWjBkcjItYUhYYTg?usp=sharing
 
Note they are in different keys and they are actually very different from each other but they all have the same thing in  common and that is they are a relaxing ambient track.
 
I have put the wave files in there as well because they do sound better than compressed tracks. There are many things going on and lots of tracks and various things here. Some electronic and others organic. The best ambient music is not all synth based but involves real acoustic sounds too. Rhythms take it out of the realm of being truely ambient. So I say no to rhythms of any form and if you do they need to be very brief. It is all about drone and texture etc...Fleeting melodies as well. I have been composing ambient music since 1983. (34 years folks!)
 
Eno Textures is hard to find and there are a couple of rhythmical tracks on that album but they are not meant to be really ambient in my opinion either more a rhythmical idea.

depends on what you mean, e.g. tracks that run 45 minutes (pink fluffy clouds) or things like EVA.. where it's an album and the tracks roll into each other..
 
FSOL are people that come to mind locally, papa nu guniea etc. 
 
pads / drones and subtle sounds, and things that are harsh you can bring in slowly. you can drown things in reverb / delay / echo etc.
 
my fav ambient tune is from an old c64 game . have a listen ... only 3 channels.
https://www.youtube.com/watch?v=-RwZED4CLkU
 
a few friends make psy trance or the goa stuff.. again "chilled out"
 

sharke
 
I'm not knocking it,  it did its job to help me sleep after all - I'm just genuinely interested in the process behind it. And I still think there's some degree of stretching out insanely long MIDI notes involved, sorry 

It would be hard to comment without knowing which specific recordings you were listing to. Maybe that was the case with what you were listening to. Maybe it was produced by some hack who just drew some notes and stretched them out and is now laughing all the way to the bank. Couldn't say without knowing what you heard.
 
The ambient stuff I'm familiar with certainly seems to me like it was written with a process in mind. Shrug.

sharke
 
...
Trouble is if it's too interesting or thoughtful then your mind latches onto it and you can't get to sleep. I really think the whole point of this "Ambient sleeping pill" music is to make something that's as innocuous and boring as possible. 

Sharke ... that's not true at all. In most cases, because KS's pieces are really long, these are not a "song" that you need to take a pill for. These are ALL, just "trips" ... it's my word for it, and in many cases, you put them on again, because they were so strong, and they have never EVER been an issue for me in the sleeping mode, specially when I have used these in meditation and sex and what not ... it's just not "invasive" music if that term can be used. It is a nice elaborate trip, but you have to get off the "knowing" what the keyboard player (or guitar player) is doing, in order to simply enjoy the trip.
 
I kinda think that you have become to DAW'd to be able to enjoy the best trip music there is out there in any form. The subtlety and the beauty of the "flow" is gone, because you have to clean it up or fix it with the DAW ... duh and duh and duh and duh ... and some forms of music are NOT that clinical, and the only way to enjoy them is to get off the clinical patterns!

JohanSebatianGremlin
 
... 
The ambient stuff I'm familiar with certainly seems to me like it was written with a process in mind. Shrug.

Not all of it. YES, when you get a chance to hear KS for the 10th time (different pieces), specially going back to "Moondawn" era, or "Mirage", there seems to be a logical process from the start to end, but the changes on it by the time you get to the end, is down right crazy, beautiful and outstanding for the enjoyment of a trip. AND, the son of a gun can do these live and did it repeatedly, and is the reason why there are so many live albums of his that were massive bootlegs in Europe ... they are all different because the same moment can not possibly be recreated, and KS is not into patching these things, simply to get you in a top ten kind of mode that you have to learn/know the stupid melody to appreciate the "song".
 
This is about "experience", not "song". And applying a "song" process and listening to it, will kill that music. Heck, you might as well say that you have classical music because it is too long and that theme repeats 45 minutes from now? (Ever wonder how they fit all these symphonies to the same length of a LP?????? Try answering that!)
 
Comparing this to the Dark Side of the Moog series, which is more "impersonal" for my tastes, is tough and difficult and some of the stuff KS did with him, made it better, but not quite the outstanding freedom and beauty of things like Cucoon and several pieces from the "Contemporary Works" series, which came out during the 90's and were likely to be considered "new age", and I think that KS wanted to have nothing to do with that ... besides the fact that fake trippers would NEVER enjoy listening to real trips instead of cardboard imaginary trips with slightly nekkid women in them!
 
There is way stronger stuff ... Frank Perry's first two albums are hardcore meditations on Tibetan Bells, and only one other person I have ever met, has been able to meditate on those like I have, however, at first it took me a couple of listens to get familiar with the sounds and what they did to me inside. That "sound" has an incredible depth, and it does things to you, that scares the living ****dipness out of people!
 
There are many other pieces, and I can list them going back to the 70's and could even include Deuter, Schoenner, Heldon, Daevid Allen and many others ... also done in different forms. 

Jeff Evans
With ambient music it also depends on how you listen to it as well. For example with Eno Ambient 4 if you parked yourself between your near field monitors and turned it up then yes you will hear all the interest and the detail much more.
 
But as Eno suggests too if say you are falling asleep and you have that music playing from a speaker set across the other side of the room and you set the volume lower then it all just meshes together and permeates across the room toward you and it becomes a more distant thing. All the detail rolls into a combined sound.
 
Hers is link to a folder on my Google drive with three Ambient tracks in it:
 
https://drive.google.com/drive/folders/0B_2Jb7O_b5BkWjBkcjItYUhYYTg?usp=sharing
 
Note they are in different keys and they are actually very different from each other but they all have the same thing in  common and that is they are a relaxing ambient track.
 
I have put the wave files in there as well because they do sound better than compressed tracks. There are many things going on and lots of tracks and various things here. Some electronic and others organic. The best ambient music is not all synth based but involves real acoustic sounds too. Rhythms take it out of the realm of being truely ambient. So I say no to rhythms of any form and if you do they need to be very brief. It is all about drone and texture etc...Fleeting melodies as well. I have been composing ambient music since 1983. (34 years folks!)
 
Eno Textures is hard to find and there are a couple of rhythmical tracks on that album but they are not meant to be really ambient in my opinion either more a rhythmical idea.

Nice tracks Jeff and very nice production too. 

The only way Klaus Schulze could sound ambient is to have the speaker almost in another room and let the sequenced sound that he makes and the truth is that his music is sequenced just merge into a more ambient sound by the time it reaches you. And that can work too. Like Eno's original Music for Airports was meant to be a bit like that.
 
But Schulze is not ambient though and not in the true sense of the word e.g. Eno's 'Apollo Moon Missions' or The Pearl or Ambient 4 for example. They are ambient albums. Or even my tracks for example.
 
Schulze also came from Tangerine Dream as well remember. Big influence from Edgar Froese. (many electronic music pioneers were also in TD the list is long) Interesting thing here is that TD are not really ambient either but they can get into it though for sure and when they do it sounds incredible. It happens in Logos for example. Jarre can do it too when he feels like it and when he does, it also can sound quite detailed and complex but the sound also tends to merge into a more ambient soundscape.

Fog
depends on what you mean, e.g. tracks that run 45 minutes (pink fluffy clouds) or things like EVA.. where it's an album and the tracks roll into each other..
 
FSOL are people that come to mind locally, papa nu guniea etc. 
 
pads / drones and subtle sounds, and things that are harsh you can bring in slowly. you can drown things in reverb / delay / echo etc.
 
my fav ambient tune is from an old c64 game . have a listen ... only 3 channels.
https://www.youtube.com/watch?v=-RwZED4CLkU
 
a few friends make psy trance or the goa stuff.. again "chilled out"
 

Not sure if I'd call that track "ambient," I like it, but there's a little too much going on melodically and rhythmically for it to be classed as ambient I think. I'm a big fan of the old SID tunes, especially those by Rob Hubbard. I used to sit and listen to those tunes for longer than I'd play the actual game. 
 
My favorite was the music from "One Man And His Droid." It's a stunning example of the 80's video game music genre, such a great composition and it's just incredible that not only was this wrung out of the little 3 channel SID chip with very limited sound shaping options, but also that the whole tune was crammed into a tiny fraction of 64 kilobytes, the rest being taken by the game itself. I get frigging emails that are bigger than that these days.