Similarly, if you ruminate on horrible things every time you stop and think, you're going to want to go on your phone late into the night until you're so tired you can go straight to sleep. If you don't even know if life is worth living, it's quite hard to avoid eating that bag of chips. I thought these were obvious, but perhaps it's not to people who've never experienced depression. Perhaps the reductionist view, as unhelpful as it is, plays into it too.
I have also experimented on myself to see whether 30 minutes of exercise improved my mood in the middle of the day. Not totally surprisingly, it _usually_ did. Not always, but usually. It never once had a negative impact.
The main problem is one of motivation/willpower. Telling a depressed person that they should exercise to feel better seems like solid advice on the surface of it, because it stands a high chance of both being true and actually working, BUT they are not in a place to usefully implement the advice. The brain gets in the way and overrides what we see as "common sense." The same way you can't tell an alcoholic to have a nice big steak instead of a beer for dinner. The person usually even knows the "right" thing to do, they are just literally unable to take the right action in the moment.
It's the same for other kinds of exercise. If you're dreading lifting weights, start by just doing some sets of really light reps. If you still feel bad, you can just stop after that, but you'll probably often find at that point that you actually feel like doing more.
Weirdly, I seem to often have my best runs/workouts with this approach, when I started out feeling like crap. Motivation is a strange thing.
On intrusive thoughts, I think identifying them early and accepting them is important to re-directing them. It takes lot of patience and practice. Don't give up on the first few tries. Just remember that eventually you will be consistent with your actions.
I am having a horrible day -> what's the least I could do to make this day better? and then do it.
Please don't aim for something you can't do. It's okay if you can get out of the bed. No need to go for run. Aim for the least and gradually increase otherwise you will not do anything due to fear.
Another skill that will help you immensely is ability to prioritize the right things. Writing down what you really should do in order and sticking it to somewhere visible helps.
In the end, no clear solution exist. For some, medication might be the best option and for others, it's lot of training to control the factors leading to intrusive thoughts and eventually depressive episode.
That is not to say that depression doesn't cause more screen time, or worse sleep.
I am willing to believe that there are people who won't feel better by going for a run or will be unable to do so, but I think there are probably also people who feel better by simply doing it, even if they don't feel like it.
Especially with depression there also seems to be an ideological battle, were some people absolutely want to believe that it is caused by chemical imbalances and medication is the only thing that can possibly work. (Again, I am willing to believe that such cases exist, but I also think there are people who would rightfully have been classified as depressed who came out of it without medication).
There's a minority of people whom regardless of their circumstances will be depressed. In this case, the argument that a person's monoamine neurotransmitters levels (e.g. serotonin, dopamine, norepinephrine) are just naturally out of whack makes sense and medications might be the best option.
Your framing assumes that there has to be a clear, unidirectional arrow of causation.
In reality, behaviors and moods are in tight reciprocity, affecting each other simultaneously. For likes of depression (or addiction) that reciprocity can narrow and narrow to the point of only moping around and doing whatever maintains the depression.
Which means avoiding depression maintaining behaviors is one of the multiple things to do in order to prevent it, or prevent a relapse, conversely it will indeed be incredibly hard to treat it with only reducing screen time.
* Lack of sleep lowers my energy and creative function. I no longer have energy to try new things, or to think out-of-the-box. The brain enters "survival mode" of just following the routine to make it to the end of day. If I let this accumulate over multiple days, the overall motivation and happiness starts dropping exponentially. This makes sense, since a lot of my happiness comes from envisioning goals and achieving them, which I cannot do if I don't sleep enough.
* If I get more than 1 night of bad sleep, I usually try to squeeze in a strenuous hike or just go cycling around the neighborhood. At least 1-2 hours, ideally 4-5. It increases the physical tiredness and makes it easier to fall asleep. It also fills in the short-term memory buffer with random stuff, that seems to have a positive correlation with subsequent sleep quality.
* Screen time generally depends on what you are doing. Routine work or clickbait browsing is bad because it doesn't make you tired and doesn't give enough short-term impressions. Playing games can give impressions, but they also raise excitement, that makes it harder to fall asleep. What works for me is to read for 30-60 minutes just before bedtime. Classic books, sci-fi, anything that is not exciting, but involves imagining what I just read about. After I'm done reading, I make a conscious effort to NOT think about work/plans/stressful topics. Thinking about what I just read usually works pretty well.
* You need to be very careful with alcohol. Moderate amount (say, 1-2 beers) makes it easier to fall asleep, but there's a high chance of waking up after 2-3 hours once it wears off. It also seems to recalibrate the nervous system, making it harder to fall asleep without drinking. So if you are having sleep problems, completely cutting of any booze for about a week shows wonderful results, despite the initial setback.
I watch my sleep cycles on my Garmin (and also track their 'body battery' thing, which I think is based on stress which is based on heart rate variability). If I have "some" drinks (eg, more than just 1 or 2 small ones), I can easily see how RHR is high and my recovery doesn't even BEGIN until say 3am. When you start tracking and watching those stats every day, you can see how different you feel when you get a good night's sleep. Vs an alcohol-aided-but-not-drunk-not-hungover sleep.
Any attempt to break it is beneficial, though it may feel like the opposite is happening.
It was more like "Emotional AIDS" - it's not that the depression made me sad, it just took away all my emotional immunity/resilience
I lost my only sibling to an overdose last year.
I lost the mother of my child to cancer in February.
Three days later, my current gf was diagnosed with cancer.
Even when I'm not sad, I'm definitely highly irritable, easily overwhelmed, lack patience...
Switch "on my phone" with "drinking alcohol" and that's literally how all alcoholics think.
Most of this is pure speculation and extrapolation from my own experience. I had a short period of time when I was drinking excessively to deal with work stress. Once I noticed, I was able to stop, because I knew it would cause me problems later on. Luckily, I never suffered any health effects or legal issues from it, mostly because it was only about a 2-3 week period when I would drink every day after work.