The question was which is better for flexibility exercises, morning or night? 🙂
You should be aware that if you are stretching for the specific purpose of improving flexibility the time of day does really matter!
Your range of motion and muscle release do vary from morning until night. You certainly should not apply your evening range of motion limitations to an early morning stretch session.
In the morning, your muscular system, joints and ligaments are very relaxed due to the fact you have been lying down all night long. When you first wake, within the first 10-15 minutes or so of moving around in the morning, your entire system is adjusting. Your muscles are adjusting to all your movement rhythms, which have been voluntarily almost static, all night. There will certainly be more movement restrictions in the morning than later in the day and regardless of whether you are a beginner or a very flexible person by nature, you need to ensure that your morning routine is not compromised by trying to achieve the same ROM flexibility that you may have later in the day.