Our 15-month-old daughter was previously

I'll check the child sleep reference and the family's profile first, then give you tonight's plan.

You've already proven the "push bedtime later" advice wrong on your own daughter — that's the data. Here's the reading and the plan.

Fork and the night this is

This is the early-waking fork in a 15-month-old, not a "too much nighttime sleep" problem. The 4-night experiment (bedtime at 9:15 PM → 45-min tantrum, 2 AM scream, 4:50 AM wide awake) is the textbook confirmation: more wake before bed produced more wake after midnight and a *worse* morning wake. Overtiredness is the cause; later bedtime is not the cure.

1. Why pushing bedtime later backfires (overtired physiology)

When a toddler's wake windows stretch past what their nervous system can carry, the body shifts from "sleepy" to "stressed awake":

• Cortisol and adrenaline rise in the evening, not just in the morning. A toddler held past her sleepiness window mounts a stress-hormone response that makes the bedtime tantrum longer, makes light sleep at the start of the night lighter, and fragments the second half of the night.
• Early-morning hours (≈ 4:00–6:00 AM) are the low point of the circadian sleep drive — melatonin is at its nadir and the body's sleep pressure has burned off. An overtired child has *less* reserve to bridge that low point, so she surfaces fully alert instead of rolling over.
• The 2:00 AM scream is the same physiology at a smaller scale: cortisol pulled her out of the first sleep cycle, and she couldn't resettle because her system was already wired.
• Late bedtime ≠ later wake. A toddler's circadian clock is set more by *morning light and consistent wake time* than by bedtime. A 9:30 PM bedtime with a 4:50 AM wake is a 7h20m night — under the 11–14 h band a 15-month-old needs, and debt compounds.

So: she isn't getting *too much* night sleep. She's getting *too little* total sleep, spread thinner by an over-tired body.

Source: AASM pediatric duration consensus (Paruthi et al. 2016) — 1–2 years: 11–14 h/24 h including naps (PMC4877308).

2. The Strategic Fork — schedule tweak vs night-wake boundary

Schedule audit (5.5h / 5.5h wake windows):

• For a 15-month-old on one nap, 5h morning / 5–5.5h afternoon is the typical range. Your 5.5 / 5.5 is in range — *the schedule itself is not broken*. What broke is bedtime drifting from 7:30 PM to 9:15 PM.
• Nap 12:00–2:00 PM is fine. Keep that anchor (see below). Do not shorten the nap to "save night sleep" — at this age that backfires the same way late bedtime does.

Night-wake boundary (the part the family advice ignored):
At 4:45–5:15 AM she is in early-waking fork territory. What happens between wake and 6:00 AM is what locks the pattern in:

• Light on → the circadian clock resets earlier. Photons through the eyelids between 4:45 and 5:00 are a powerful "this is now morning" signal. The clock then wants to fire the wake again at the same time tomorrow.
• Out of crib / interaction → a learned sequence. After a few mornings of "Mummy appears, lights on, maybe a bottle," the body learns that 4:50 AM is when the day starts.
• Bottle at 5:00 AM → ghrelin / hunger timing shift. A small feed at that hour, repeated for days, nudges the body's hunger hormones to expect calories at 5 AM. Skip the calorie at 4:50 AM and the body stops asking for it.

So: schedule tweak alone won't fix it; behaviour at 4:50 AM alone won't fix it. You need both — earlier bedtime to clear debt, and a strict "still night" response before 6:00 AM.

3. Tonight's two concrete moves

Move 1 — Bedtime back to 6:45–7:00 PM tonight (not later).
The accumulated sleep debt from 3 weeks of 4:50 AM wake + 4 nights of 9:15 PM bedtime needs to be paid down. Pull bedtime to 6:45 PM; if she's rubbing eyes or yawning during the wind-down, take her at 6:30. Keep the bedtime chain the same (pyjamas, teeth, one book, crib awake but drowsy). Aim for 11–12 h in bed overnight as the runway to clear debt.

Move 2 — Anything before 6:00 AM is middle-of-the-night, not morning.

• No lights, no eye contact, no talking, no lifting out of the crib.
• If she cries, a single calm pat and a quiet "shh, still sleeping" — same voice, same touch, same darkness. Then you leave.
• No bottle, no milk, no snack at 4:50 AM. If she is genuinely hungry, that is a signal worth raising with the GP at a routine visit — not a 5 AM feed to lock in.
• Blackout curtains or a darker shade on the window. The point is to keep the pre-6:00 AM light out so the clock doesn't reset.
• OK-to-wake clock or visual cue set to 6:00 AM or later, not 5:00. Until the clock says it's morning, it isn't.

4. Weekly anchor

> Morning wake time at 6:30 AM, lunch nap 12:00–2:00 PM — every day for the next 7 days, including weekends. Bedtime falls between 6:45 and 7:00 PM automatically.

That's the sentence to hold. The morning wake time and the nap time are the two fixed points; bedtime is what it lands on when you back-count 5–5.5 h from nap end plus the wake-up runway. Same wake time weekend and weekday — a stable morning clock does more than any bedtime trick. If she wakes before 6:00 AM, the Move 2 response runs; if she wakes at 6:30 or after, you get up and start the day normally.

I'd expect the 4:45 AM wake to soften toward 5:30–6:00 over 5–7 days if the anchor holds. If after 10 days of the 6:30 AM anchor + 7:00 PM bedtime she is still surfacing before 5:30 AM and cannot resettle, that is the moment to bring this to the GP — bring the timeline (when it started, the late-bedtime experiment, the 5-hour response, what's happened since) so the GP can look for the next layer (regression, iron, sleep-breathing).

One more thing

Do not use the late bedtime as a punishment, and do not let the family push you back to "run her ragged in the living room" tonight to fix tonight. Her body is in debt; what clears debt is *more* sleep, not less. If grandparents are watching her for a block this week, write the bedtime and the 6:00 AM rule on a note on the fridge.

Want me to set a daily reminder for the 6:30 AM wake and the 12:00 PM nap start so the anchor doesn't drift over the next week?