You asked

Why are tweens and teens so tired?

In order for anyone to function properly during the day, they need a good night’s sleep. For tweens and teens that means getting about nine or ten hours sleep each night. If you have a growing tween at home, it’s unlikely that he is getting that amount on a regular basis. In fact, recent polls suggest only one-fifth of teens get the recommended amount each night.
Teen’s internal clock
The reasons for teen’s strange sleeping habits is puberty. Young people’s internal clock is altered during puberty and melatonin is now secreted later in the evening which means teens don’t get tired until later in the day.
Teens are growing at an enormous rate so they still require a lot of sleep. This means they need to sleep later in the day which means it can be hard to get them up for school in the morning!
Effects of sleep deprivation
A lack of sleep during the important teen years can have serious side effects. Concentrating in school will be harder as will trying to get homework finished. This can lead to poor performance and low test scores.
A lack of sleep also leads to depression and mood swings,
Sleep deprivation can also make a teen more susceptible to acne and other skin problems and can also lead to medical problems such as sleep apnoea and anaemia. 

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