Why does overlap matter more than the nap itself?

Because with two babies, a nap that does not coincide with the other baby's nap is not a break. Two 90 minute naps that miss each other give you nothing; two 40 minute naps that line up give you 40 minutes. Parents of twins are usually optimising the wrong number.

Should I wake a sleeping baby to line them up?

In the morning, yes, and it is the single most effective thing you can do. Waking the later twin so both wake within about half an hour puts both wake windows on the same clock, and the naps follow from there. During the day the opposite applies: let a nap finish and adjust the next wake window instead.

My twins are different sizes. Should they have the same wake windows?

Not necessarily. A smaller twin, or one who was more premature, often needs a slightly shorter wake window than their sibling. Run each baby through the tool with their own times rather than assuming one schedule fits both.

What if one twin dropped a nap and the other did not?

That is normal and it usually lasts a few weeks. Keep the baby who still needs the nap on it, and use the overlap number to find where the remaining shared window is, which is often the middle of the day rather than the morning.

Does this work for a baby and a toddler?

The arithmetic does, if you enter each child's own wake window. With more than about a year between them the overlap is usually one nap rather than several, so the useful output is which nap to build the day around.

How accurate are the wake windows it uses?

They are age averages, and your babies' real windows are better data. If one twin reliably falls apart 20 minutes before the average, treat that as their window and enter their times accordingly, because the overlap will be more accurate for it.

Twin nap overlap finder: when will both babies be asleep?

Twin nap overlap finder

Their age

Baby A last woke

Baby B last woke

Worked out in your browser. Nothing you enter is sent anywhere.

The number nobody calculates

Every baby sleep tool asks about one baby. With twins that is the wrong unit. What determines whether you get a break is not when each baby sleeps but whether the two naps intersect, and two long naps that miss each other are worth less to you than two short ones that line up.

So the useful question is not "when is the next nap", it is "when are they both down". That is what this works out.

Why the fix is in the morning

When the naps miss, the instinct is to push the later baby's nap earlier. That usually fails, because a baby who has not been awake long enough will not go down, and you end up with a fight instead of a nap.

The lever that works is the wake-up. Both babies' wake windows start when they wake, so moving one wake-up moves everything downstream of it. Wake the twin who is running late, by 15 or 20 minutes at a time over a few days, and the naps converge on their own. That is why the tool gives you a morning number rather than a nap time.

What it does not know

It uses age-average wake windows and nap lengths, and your babies are not averages. It also cannot see a tooth coming through, a growth spurt, or the twin who has decided that 4am is morning now. Treat the output as a starting position for the week rather than a timetable, and trust a pattern you have actually observed over the one the tool assumes.

Where the app fits

Doing this once is arithmetic. Doing it every day, for both babies, while holding at least one of them, is why it stops happening by the second week. Multibaby logs each wake and nap for both babies by voice and shows the overlap as it happens, so the number is there without you working it out at 6am.

Quick answer

The number that matters with twins is not when each baby naps, it is how long they are asleep at the same time, because that overlap is the only rest you actually get. Enter each baby's last wake time and their age, and this works out both nap windows and the part where they intersect. When the naps miss each other it tells you how much earlier to wake the first baby tomorrow, because the fix is almost always at the start of the day rather than at the nap.

How to use it

Pick their age. Choose the age band for your twins. It sets the wake window and typical nap length the calculation uses.
Enter each baby's last wake time. The time each twin last woke from sleep, including this morning's wake-up. Nothing you enter leaves your browser.
Read the overlap, or the fix. If the naps intersect you get the window and its length. If they miss, you get the number of minutes to move the earlier baby's wake-up tomorrow.

Frequently asked questions

Why does overlap matter more than the nap itself?: Because with two babies, a nap that does not coincide with the other baby's nap is not a break. Two 90 minute naps that miss each other give you nothing; two 40 minute naps that line up give you 40 minutes. Parents of twins are usually optimising the wrong number.
Should I wake a sleeping baby to line them up?: In the morning, yes, and it is the single most effective thing you can do. Waking the later twin so both wake within about half an hour puts both wake windows on the same clock, and the naps follow from there. During the day the opposite applies: let a nap finish and adjust the next wake window instead.
My twins are different sizes. Should they have the same wake windows?: Not necessarily. A smaller twin, or one who was more premature, often needs a slightly shorter wake window than their sibling. Run each baby through the tool with their own times rather than assuming one schedule fits both.
What if one twin dropped a nap and the other did not?: That is normal and it usually lasts a few weeks. Keep the baby who still needs the nap on it, and use the overlap number to find where the remaining shared window is, which is often the middle of the day rather than the morning.
Does this work for a baby and a toddler?: The arithmetic does, if you enter each child's own wake window. With more than about a year between them the overlap is usually one nap rather than several, so the useful output is which nap to build the day around.
How accurate are the wake windows it uses?: They are age averages, and your babies' real windows are better data. If one twin reliably falls apart 20 minutes before the average, treat that as their window and enter their times accordingly, because the overlap will be more accurate for it.