This is a common question for many women who have both curly and straight hair. It's not a bad thing, but it does require attention and care to keep both types of hair looking their best.
Your genes determine your hair type. Most people have about 4 different curl patterns in their hair, ranging from wavy to curly, loose and tight.
But the gene that controls your curls is not totally dominant, which means it can be dormant for a while until activated.
The hair up top gets more exposure to the sun and heated styling tools than the bottom hair.
This can break down the cuticles in that layer of the hair, causing it to lose its curl.
Another factor that can cause the hair on your head to become straight is brushing it too much.
Often, people who have both curly and straight hair on their heads will brush their hair in the morning before they shower, as it will help to detangle the strands.
Prolonged exposure to the sun or extreme cold weather can also lead to your hair becoming dry and brittle.
This will make your hair more likely to straighten, especially the top layer. If this is the case, then you might want to consider a cut that will reduce the weight of your top layer and draw your curls up.