Babies and toddlers have many different signs that cue tiredness, including the following: eye rubbing, ear tugging, back arching, tuning out (getting that "1000 yard stare" as my sister calls it), scratching, squirming, losing balance and coordination easily, and more...not just yawning.
According to Tracy Hogg, the "Baby Whisperer", it's best to get your baby to sleep as soon as you see the first yawn and at the latest the third. It's best to learn your baby's sleep signs and try and get them down for a nap or to bed as soon as they start to signal it's time to go to sleep. Sometimes, missing that window means meltdowns or putting your child to bed "overtired". In these cases, the child gets a second wind and starts to get hyperactive.
No one wants to miss that window.
Some babies are a bit more subtle in showing they're sleepy and then all of a sudden, they are super cranky and it's difficult for mom and/or dad to get their little one settled. In these cases, it's important to know what is the amount of time your baby can be awake before it's time to sleep again. Infants, for example, can only tolerate 45-60 minutes before their body is saying "It's time to rest!" Older toddlers can go up to anywhere from 3-6 hours before needing sleep.
So, what to do?
Start by putting your child down a little too soon than too late. The calmer, more relaxed that they are, the easier it is for them to fall asleep. As soon as your baby is showing signs of tiredness, give him a little time to wind down and get ready to transition from wakefulness to sleeping. It's a good idea to give your baby a bedtime and naptime routine to make this easier. This can include singing a lullaby, bath, reading, quiet music, etc. The more closely you follow this routine, your baby's brain and body will start to associate that list of activities with going into "shutdown mode."
Follow (as best as possible) a consistent nap and bedtime schedule. When your body and brain can predict when its sleep time, the easier it will be to fall asleep. Give your child the same time, same place, same routine -- things will go much more smoothly.