So I’m 5 days back on keto after a little over a year hiatus. When doing keto before fasting came relatively easy for me. Well 36 hours was my longest so I dont know if longer would have been the same. I tend to naturally IF even when not following the keto WOE. I usually have a hard time stomaching solid food in the mornings and tend to not start feeling hungry until around lunchtime. And even then non keto me could just grab a bag of chips or something and be fine until dinner. So yesterday afternoon I made the conscious decision to start 16/8 IF daily with the goal to eventually build up to EF. It’s like a freaking light switch went off in my head. I woke up ravenous and I’ve been hungry all day. This is probably the first time I’ve woken up hungry in years. Could it really just be all in my head because I put a label on how / when I was going to eat? Or could there be something more physical to it?
Physiological vs psychological "hunger"
Starting any fasting only being 5 days back on the wagon to me means it’s probably not real. I’m pretty anti-fasting these days but it’s most likely still some sugar pushback.
Don’t know whether it’s one or the other. However, physiologically speaking feelings of severe hunger are generally a response to glucose/insulin ups and downs. So I’d guess that as long as you remain sub-20 grams of carbs, stay in ketosis consistently, and eat when you’re hungry - it will pass. Yes, I agree with @lfod14 that fasting is probably premature. If it were me, I’d be logging it all. It would likely be helpful for others to know that after a year’s hiatus, going back to keto might trigger a carb withdrawal episode of epic proportions. But that after x days of hell, things got better again. 
I would tend to agree with you both that forcing a fast this early wouldn’t be the best idea. But I wasnt really thinking about it that way. It was more like "this is how I eat anyway so why not go ahead and start tracking it ". I’m one of those annoying keto people who tracks and logs EVERYTHING. What I eat, when i eat, weight, body measurements, sleep patterns, energy levels … hell last time I was even taking pictures of the top of my head once a week because I suspected I was losing more hair than normal.
I ended up not eating this morning because I didnt plan for enough time to make anything before work but had I had plenty of time I definitely would have eaten and not tried to force anything this early into it.
You could just ‘tough it out’. You’re already 5 days in, so it can’t take all that much longer to get through it. Although it will be interesting to know just how long it takes. I’m 3+ years into keto and seldom feel hunger or satiety, which is why I weight everything and eat to macros. You may find the following of interest regarding glucose (and by association insulin):
Odd to me as I never had this but if you are hungry, eat. Or do whatever you do in this situation, I wouldn’t keep anyone from forced fasts even if I had the power unless it would save their lives.
I naturally do IF and no matter what I do (I have a lot of experience of doing various things eating wise, plus different waking times and exercise), I am full in the morning… But if I ever feel hunger, I eat. I would expect a very small meal in that situation anyway and this odd morning hunger would surely pass with time. I try to understand my body but it has its own things. I trust its signs.
I do ALL that! If you don’t track you’re just guessing blind. If I was a tracker a couple years ago I’d still have a good metabolism and wouldn’t have gained/lost the same weight a bunch of times and not known why. I’ve also been tracking my hair… still in my 30’s, even if barely. Too young for that shit! Although I suspect I may have done that to myself with some other… stuff 
Well @Shanda, I would love to answer your question, if I could figure it out for myself. 
I tend to find myself fasting when I don’t plan on it, and I realize it’s been over 24 hours since the last time I ate. But if I plan to fast, I could start to feel hungry by lunchtime. 
I don’t know how much of this is physiological or psychological, or just illogical, but it tends to affect me differently every time. I think I do best when I’m busy. If I don’t have time to think about it, then I don’t. I know this seems obvious, but it’s ridiculous to me how simple it is.
I know food doesn’t have control over me, and that I don’t need to eat anytime my stomach growls, but if I allow myself to think about it, then it feels like I have no control. Does that make sense?
(Wow! A lot of deep psychological advice here, huh?) 
Another thing that tends to help me, if I’m feeling a bit hungry, sometimes just drinking a little water makes that feeling go away for me. Then I can go back to focusing on other things.
Good question! I hope someone with more insight comes along and does a better job answering it than me.
This absolutely makes sense! And last time I did this I had gotten pretty good at recognizing clock hunger vs my body actually needs something to eat. I came into this with the mindset that the first month or so would be easier this time around because I already knew what to expect and had developed strategies for dealing with the pitfalls. I figured I just needed to concentrate on not making the same mistakes I did last time which inevitably contributed to me going back to my unhealthy habits. But instead I’m finding all new issues popping up.
Recent experience suggests to me that more or less hunger is mostly about fat adaption. I believe my fat adaption has improved over the entire 2 years I’ve been doing this. A year ago I tried carnivore for 60 days and stopped because hunger went crazy. Now I’m trying it again and things are different. No hunger pangs even though I’m deficiting 350 calories per day (to get rid of a few lbs of body fat). I believe fat from food goes to stored fat and fuels the body from there. May be things are different this time because drawing down fat stores is more routine and may be my body agrees that I have a few more pounds of fat than I need. It’s no surprise to me that it might have taken 2 years of low carb/keto to get to this point. Along the way I did plenty of low heart rate training, which favors burning fat. I believe the exercise activity was important for speeding up the fat adaption process.
I use to like to monitor my glucose with a blood meter and and peek at ketones constantly (obsessively), like thousands of dollars constantly.
What I began to understand is that hunger signaling got really strong when glucose was low.
But there is another thing I noticed between hunger and that is; nausea (w/headaches/dizziness/weakness/heart palp), feeling hungry was ok (psychological?) but feeling nauseous (when you have nothing to vomit and dry heaving?) was not, that tells me it is not psychological? That means it is time to eat?