Looking down into the pushchair I saw a young baby drifting off to sleep, and then flick his eyes open. At one point one eye was open and the other was shut.

“What a lovely baby,” I said. “How old is he? Your first?”

“Yes. Four weeks. But he hasn’t slept through the night once since he was born. I don’t know what to do – do you?”

This mother’s sleeplessness explains why Google is so often asked: “Why don’t babies sleep at night?” To find out the answer you need to go back to the beginning.

If you’re a mother, when you conceived your baby, they brought a number of changes into your life – and theirs. For you there was a new lifestyle (healthy food, physical changes, no drinking) and a huge responsibility. For your baby, it was the beginning of a life where their only guide for nine months would be their genes and the environment in your womb.

Your baby will have rapidly developed a sense of time from the rhythms in the womb, including your heart beat. They will also have had two sets of timing genes. The first set ensures they sleep, just like other animals. This very large set of genes is very robust indeed: every baby sleeps, and no disease or experience will change that. The second set of genes creates their 24-hour sense of time throughout the day, including waking and sleeping.

Genes are, of course, a bit of a lottery. Although you may have a very distinct sense of sleep and the timing that suits your day (and night), your baby’s genes are a mixture of yours and your partner’s. Babies are different and have different sleep patterns, so the chance that their sleep patterns will match yours is very small.

During pregnancy, your baby experienced the rhythm of night and day, sleep and wake, only through you. These messages are confused in pregnancy because your sleep patterns change for many different reasons (physical changes, baby’s movements, needing to urinate more at night, tiredness and not getting enough sleep yourself). Your baby may have been soothed by the experience of you moving around during the day, and more active at night when you rarely moved. Throughout pregnancy, your good sleep will have helped your baby. We know that around the 32nd week of pregnancy, your baby sleeps too.

That’s one of the reasons why sleep needs to be a high priority for every pregnant woman; another is that sleep is vital for giving birth. Although being fatigued during pregnancy is normal, women who sleep less than six hours a night, on average, have longer labours and are 4.5 times more likely to have a caesarean delivery.

At birth the need to feed dominates a baby’s world – and that of its parents. Feeding is hard work for a baby, and makes them tired – so they sleep, rest and recover. Then the need to feed comes again, often in a pattern of every four hours or so. Babies have no established sense of night or day, and they tend to have cycles that are far shorter than 24 hours long. This is because their timing systems for the 24-hour day are not yet fully formed at birth, and they won’t function consistently until a baby is about two to six months old.

So the short answer to the question is this: babies don’t sleep through the night because they can’t.

At between two and six months, a baby’s timing systems should develop so that their sleep has clear patterns. A very recent scientific discovery has shown that although a baby’s sleep, like an adult’s, is divided into a time of dreams (REM – rapid eye movement sleep), and quiet deep sleep (non-REM), a baby has far more REM sleep than an adult. It appears this is necessary to consolidate a baby’s rapid learning about the world, including their understanding of night and day. And for a baby, sleep is vital for brain development in other ways too.

That’s all very well I hear mothers (and partners) say, but what can we do to make our baby sleep at night? We’re desperate. Do we let them cry or not? Does breastfeeding help? How can we make sure they are safe?

Most sleep advice for babies applies to the whole family. The difference is that babies have to learn the timing of day and night only from signals in the environment.

Sunlight is the strongest environmental signal of all. Going outside is vital for your baby (and you) in setting your internal clocks to the same time. So the darker the bedroom, the better it is for sleep. In contrast, the light-emitting screens of televisions, phones or computer devices used before sleep, or “night lights” during sleep, are not a good idea.

Night is signalled by a fall in temperature, so cooler bedrooms are better. It is important to establish a regular pattern of waking and going to sleep, so the routine becomes familiar and pleasant.

Breast milk contains the hormone melatonin, which signals your sleep time to your baby, just as it did during pregnancy, if you decide to breastfeed. In the early weeks a baby is likely to doze off for short periods during a feed. Carry on feeding until you think they have finished or are fully asleep.

Guidelines are available for keeping babies safe while they’re asleep: Helping Your Baby to Sleep (NHS) and Keeping Your Baby Safe (NIH) offer good advice.

Also, trust yourself: if you can’t bear your baby’s distress when they still wake up crying at 14 months, go to them.

And try not to worry. In the end, all of us will sleep.