Why am i still tired after sleeping eight hours?

Need more: It can be as simple as you need more than 8 hours of sleep. This might be normal. Alternatively, you may have a medical / psychiatric condition that causes you to feel tired or increases your need for sleep. Some disorders (e.g. Depression, sleep apnea) causes sleep to be less restorative, less efficient. This will increase your need for longer sleep time.

Sleep apnea: Another possibility coud be sleep apnea. Persons often spend plenty of time in bed, but never truly enter deep, resotrative sleep when they have sleep apnea. There are other serious consequences of sleep apnea besides tiredness as well. Other medical conditions are possible such as low thyroid too. See your doctor for evaluation.