To answer this question you need to look at the history of the domesticated dog also known as Canis lupus familiaris.

It is a natural instinct instilled in the behavior of puppies that carried through with the domestication of the species.  In the wild it was crucial for the survival of mom and babies to huddle while mom searched for food.  The new born babies huddle for warmth, slept slowing their metabolism and remained quiet to draw less attention from prey animals looking for a easy meal.

As the babies get older and they can better regulate their own body temperatures they become more active.  However, the instinct of sleeping while mom is away remains until the babies are nearly weaned from nursing which is 5 to 6 weeks of age.  At this point the amount of time spent sleeping starts to shift to learning and exploring, finding food and water, and interaction with litter mates.  This interaction with litter mates teaches a multitude of life long socialization and behavior skills such as bite inhibition (in other words that biting hurts and is really inappropriate), they also learn to share food, toys and other common items.  During this time they are also learning to socialize with humans, learn what sounds are normal and what sounds they need to be more alert on.  This period of time is crucial to the development of the mental wellbeing of the puppy and is very important during weeks 5 to 8 of the process.  This is the main reason puppies should not be removed from the litter and placed in homes before 8 weeks of age.  With that said a responsible breeder with experience should be able to determine when a individual puppy is ready to thrive in a new home away from siblings and for some puppies this can take longer then 8 weeks.

Once a puppy matures and is no longer dependant on mom and siblings.  They take the skills they have learned and apply them to new surroundings, making new friends (dogs, cats and other animals in their new homes), and begin to adjust to their new environment.  It is very important for a new puppy especially 8 to16 weeks old to slowly extend the amount of active time awake verses rest periods.  As a human you can not go from sitting behind a desk to running a marathon over night your body can not take on this amount of change and will collapse..... same applies to puppies.  With puppies this excitement of new space to explore can cloud their best judgment and this is where we need to step in and let them know its time to rest.  Usually its best starting out at 15 to 20 minute play sessions then rest periods of a hour or more.  You can start off a rest period by holding and bonding with your puppy in your lap, sort of a wind down period before a nap.  When your puppy first wakes and potties .....they are full of energy and ready to play....this is really not the best time to try to pet and hold the puppy because the energy level is to high and they will most likely fight to get loose and play.  You can still bond with your puppy by interacting in their play time.  Teaching them their name and offering a small treat reward when they come to you.  This is early association training that coming when called is a good thing and is rewarding.  Common mistakes during this time is chasing a puppy down and reprimanding a puppy for not coming when called this also is association training that creates a bad behavior.  The puppy learns that hearing his name means something bad might be following it so run and hide.  No matter how angry you are when you finally get your puppy in your hands praise and reward this re-enforces coming when called is a good thing.

It is also very important to maintain nutrition and hydration of a puppy.  Make sure the puppy has water available at all times as well as offering food every  2 to 4 hours if you are uncomfortable with leaving food available all the time.   Free Feeding is the term given to leaving food available all the time and in most cases its a learned behavior to eat when they are hungry because they know its there when they feel the need to eat.  Some puppies/dogs when fed on a schedule can develop a gorging behavior resulting from a fear of not knowing when the next meal might arrive.  So they eat it all very fast whether they are that hungry or not and can also cause the development of food guarding and guarding of other items in the house.

To sum it all up the first 6 months of your puppies life is most important and is the beginning mold for the lifetime you spend with your best friend.  This time period is the foundation for the building of your relationship and how the dog responds to you as their leader.  Keep in mind when getting your new puppy ... you will get back what time and energy you put in at the beginning.... Time not spent in the first 6 months is one of the leading causes of dogs ending up in shelters.