We used to have a baby that cried a lot. We couldn't quite 
understand why. Every time she cried we kept on distracting or 
comforting her hoping she'd grow out of it, as books on baby care 
promised. 
Stressed but unrelenting, I
 was determined to find a solution. Article after article, book after 
book I found the roots of our problem: our baby was extremely 
overtired. 
From all the research I've done I 
learned three  important things: (1) a 4-month-old needs to sleep 14-15 
hours a day. (2) a 4-month-old cannot stay awake for more than 1.5-2 
hours. (2) A 4-month-old  needs to go to sleep for the night at 7pm!
 Having read four books on infant sleep I realized this was the three 
things they all agreed on. Understanding this made a huge difference for
 us and our baby. 
The cause of our crying was 
very clearly explained in the book by Marc Weissbluth Healthy Sleep 
Habits, Happy Child. What often happens is when the baby wants to sleep,
 the parents interfere. The baby starts crying because she wants to 
sleep. What do the parents do? They try to distract her with toys or by 
taking her to a different room, or some other type of stimulation. My 
mother-in-law, for instance, was convinced that the baby wants a bath, 
while the bath, being a very stimulating experience for my baby, was the
 last thing she needed.  As a result of our misreading of her cues, our 
baby grew more and more tired and her cries escalated into desperate 
screams. Our baby was a lot smarter than we were. She needed to sleep 
and she kept on trying to tell this to us. We were ignoring her pleas, 
what else could she do other than scream? Instead of trying to distract 
her, we should have respected her need to sleep. At the first signs of 
drowsiness we should have put her to bed. 
Ok,
 we figured out that our baby needed to sleep. But bedtime has always 
been associated with crying. This is where the bedtime routine came in. 
(BTW, this is another one of those things that all baby-sleep books as 
well as scientific papers agree on).  As soon as our baby showed signs 
of tiredness, I decided that we should put her into her sleeping back 
and into the crib, so she'd have a chance to rest and fall asleep. Of 
course she doesn't fall asleep by herself yet (wishful thinking), but 
hopefully one day she'll learn. We let her stay in her crib by herself 
until she starts crying. Then we come in and comfort her, and when that 
is not enough, we pick her up and rock her to sleep.