Sleeping can affect curly hair, potentially leading to a loss of definition and causing curls to become flatter or less bouncy, but it generally does not completely straighten curls unless combined with other factors or styling methods. Here's why:

1. Friction: When you sleep, the contact between your hair and the pillow, especially if it's a cotton pillowcase, can create friction, causing curls to lose their shape and become more prone to frizz.
2. Pressure: Laying your head on a pillow applies pressure to your hair, which can compress and flatten curls, especially if you tend to sleep in one position for a long time.
3. Absorption: If your hair is not fully dry when you go to sleep, the moisture can be absorbed by your pillow, potentially altering your curl pattern and causing it to look less defined in the morning.

However, there are ways to mitigate these effects and help maintain your curls overnight:

• Pineappling: Gather your curls into a loose, high ponytail on top of your head, using a scrunchie made of silk or satin to minimize damage and friction. This method helps preserve your curl pattern.
• Silk or Satin Pillowcase: Switch to a silk or satin pillowcase which reduces friction and helps hair glide smoothly, minimizing frizz and breakage. These materials also help retain moisture in your hair.
• Sleep Cap or Bonnet: Wear a satin or silk sleep cap or bonnet to protect your curls from rubbing against the pillow and to maintain moisture.
• Prevent Tangles: Loosely braid or twist your hair before bed to reduce tangling and maintain some structure to your curls.

By adopting these habits, you can wake up with curls that are closer to their original shape and definition, rather than finding them significantly straightened or flattened.