A disclosure before we go further: I develop soup cleanse products, which is a reason to read the evidence sections closely rather than take my word for it. Where the research is thin, I’ve said so.
The timeline that gives it away
Caffeine withdrawal headaches follow a predictable clock. Onset runs 12 to 24 hours after the last dose, peaks somewhere between 20 and 51 hours out, and resolves within two to nine days. About half of people who stop caffeine abruptly get a headache from it, and the mechanism is mechanical rather than mysterious: caffeine keeps blood vessels in the brain slightly constricted, and when it’s gone, they widen. That rebound in blood flow is what you feel as a headache.
Line that timeline up against a cleanse that starts with your normal morning coffee and nothing after: last real dose is roughly 8 or 9am on day one, symptom onset lands 12 to 24 hours later, which is the evening of day one into the morning of day two, and peak intensity hits 20 to 51 hours out, squarely in the middle of day two. That’s not a coincidence. That’s the same clock caffeine withdrawal runs on for anyone, cleanse or no cleanse.
What else is stacking on top of it
Caffeine withdrawal explains the timing, but it’s rarely acting alone. A soup cleanse also drops your sodium intake substantially, since broth-only eating removes most of the sodium a normal diet supplies from solid food. Low sodium causes its own headache, and it layers directly on top of the caffeine one rather than replacing it. If you haven’t salted your broth properly, day two is where both bills come due at once.
What actually helps
Keep some coffee in the routine, at least for your first cleanse. Nothing about the straw test rules out black coffee, and there’s a real case for not stacking a caffeine withdrawal headache on top of everything else a first cleanse already asks of you. If you want to go caffeine-free too, taper in the days before you start rather than stopping cold on day one.
Salt your broth properly, every batch. This is worth repeating because it’s the detail people skip. A pinch of sea salt in your water and a properly seasoned broth are doing real work here.
If the headache still shows up, it tends to ease by day three, which lines up with where caffeine withdrawal symptoms peak and start to fade. That’s useful to know in the middle of it: it’s not a sign the cleanse is going wrong, and it’s not going to be the whole three days.
When it’s not this
A headache that gets worse rather than better, or that comes with genuine dizziness or a racing heart, isn’t caffeine withdrawal anymore. That’s the same stop-if-unwell threshold that applies to the rest of a cleanse: end it and eat a proper meal.
Most day-two headaches aren’t a verdict on the cleanse. They’re your coffee habit, settling a bill it was always going to collect eventually. The cleanse just happened to be what moved the due date up.