Most parents are more than familiar with their own sleep problems that occur after a frazzling day. The truth is, even though babies don't have to worry about nasty bosses, paying bills, and a water heater than needs to be replaced; they, too, can get frazzled and anxious. They, too, are likely to have trouble going to sleep, or staying asleep, when their day has not been one during which they have felt calm and secure.
While adults' threshold for becoming anxious is much higher than that of babies, the principle of anxiety causing sleep problems is the same. One of the biggest mistakes any parent can make is to assume that, because a baby "only eats, sleeps, and plays all day" , stress and anxiety can't play much of a role in sleep problems. The truth is that we, adults, generally take for granted that we are safe and secure during our days. Healthy adults don't get anxious unless there are those stressful worries, such as bills, sick family members, or any number of other things. Babies, on the other hand, require the close-to-constant reminders that they are, indeed, safe and secure. The perfectly happy and comfortable baby who feels safe and secure as he plays will suddenly feel less sure when hunger sets in. In other words, a baby's days are filled with "ups and downs" when it comes to feeling safe and secure. Too many "downs" will lead to a particularly frazzling day.
With their immature central nervous systems, their helplessness, their inability to understand language, and their general need to have a sense of equilibrium; babies can also be prone to becoming over-stimulated too. Each baby has his own disposition, and some babies are more sensitive to "too much going on" than others; but all babies, to some extent, can suffer a sense of anxiety when life gets to hectic. Whether it's too many bright lights, too many different loud noises, being brought around to too many unfamiliar places, or being passed around by too many people; babies can get frazzled and anxious from over-stimulation. Even too much "active play" can lead to a baby's feeling frazzled; because although a baby may enjoy such play, there can be a sense of feeling a little too unsure about what is coming next. All of these things that can contribute (sometimes in small ways, sometimes collectively) to a baby's having a frazzling day are likely to also contribute to his having sleep problems. Many of a baby's potentially "frazzling" activities/situations during any day may actually be pleasant experiences. That does not, however, always translate into being experiences that are most likely to contribute to a sense of sureness and security.
Sureness and a sense of security come from those quieter, surer, calmer, interactions between parent (usually mother) and baby. A sense of security comes, too, from knowing that Mommy is always there to respond to needs and distress. For a baby, a sense of security also comes from having his needs met and from experiencing a caretaker's calming, gentle, touch and voice. Until a child has grown enough not to experience some of the more active parts of some days as frazzling, it's important that he experience enough time feeling super-secure and super-safe to balance off some of the more frazzling times in his day.
In general, most of cannot make the leap from feeling anxious to being asleep without having some time in-between to bridge the gap. Some adults may simply stay up until they're ready to "pass out", but that doesn't work for babies. Babies get over-tired, which means they get yet more distressed. Feeling yet more distressed means they feel that much less safe and secure. Even when a baby has been over-tired and distressed, once he does "pass out" he is likely to have a fitful sleep and wake during the night.
Babies who feel safe and secure are babies who are generally happier babies, who are also easier to care for in general. They're usually happy with whatever is going on, and being putting in their crib for the night can just seem like one more pleasant part of their day. Some secure babies will have those frazzling days that make them feel a little less secure for a brief time. Some babies, however, will live all their days feeling a little anxious because some parents may not be quite as skilled as others at helping their baby feel safe and secure. Essentially, whether a baby feels a little less secure on a particularly frazzling day or is a child who lives feeling a little less secure, the root of the problem is that when any of us is anxious our brain chemicals change in a way that is not conducive to getting a good night's sleep. To make the problem worse, a baby who does not get the right amount of the right quality sleep will begin the following day at a disadvantage, when it comes to feeling safe and secure.
When it comes down to it, babies require very little in this world. With the exception of getting their nutrition, the need to feel safe and secure is the most crucial need any baby has. When such an important need is overlooked, or when parents are not skilled in meeting that need, that is certainly a problem big enough for anyone to lose sleep over.