The iPhone X’s Face ID feature has successfully recognized user faces under a variety of circumstances, including while sporting glasses, makeup, or a beard. However, several users have reported that the advanced facial scan fails under one particular condition: When a user first wakes up in the morning, Slate reported.

The problem seems to lie in facial swelling, or the non-serious inflammation caused by allergies, water retention from dehydration, or fluids pooling up in the face while a person is lying down. While this doesn’t radically change a person’s appearance, it does appear to give Face ID a problem, at least for some users, according to Slate.

SEE: IT leader’s guide to the future of artificial intelligence (Tech Pro Research)

Face ID works by mapping a user’s face in a similar way to how Touch ID maps a fingerprint. It doesn’t store an image, but makes a map of the face using data points. When those data points match up with a face, it unlocks the phone. The method offers greater security than Touch ID, but it still can be hacked.

The problem may be less with the user’s face shape than with swelling around the eyes, Marc Rogers, CSO for ScaleFT, told Slate. It appears that Face ID “assigns a higher value for certain parts of the face,” he said. For example, a mouth can be covered and Face ID will still work, while a nose cannot. It appears that Face ID needs open eyes to work, Rogers told Slate.

The issue may be frustrating for iPhone X users who spent at least $1,000 on the device. But at least they aren’t completely locked out of their phones: If Face ID doesn’t work, users are prompted to enter their passcode.

This isn’t the first time that Face ID has experienced problems. Some researchers used a 3D printed mask to hack the technology, and it experienced faulty rear camera issues. These issues could further increase fears around biometric security, as it becomes a more popular option for unlocking critical devices and information.

The big takeaways for tech leaders:

  • A number of iPhone X users have reported that the phone’s Face ID feature does not recognize them when they first wake up, possibly due to facial swelling that occurs when someone is lying down.
  • When Face ID does not recognize a user’s face, they still have the option to enter a passcode and unlock their phone.