Why do we feel tired after we sleep for a really long time, instead of waking up fresh?

Its not how long we sleep to make it a refreshing sleep.

Its because we have so called fragmentations of sleep all through out the night, caused by sleep apnea.

For as short as 10 seconds of breathing cessation our brain have a hard time to go on a deeper stage of sleep, that causes fragmentation and lightening of sleep.

If this happens throughout the night our sleep efficiency will decrease and waking up not feeling refreshed.

there are couple of reasons for not waking up fresh after taking a very long time sleep but few are of them i answer below,

reason is that you have take a deep sleep or a disturbing sleep like bad dreams or wrong hours of sleep may not give you a proper sleep , many times person sleep with electronics gadget which radiates very harmful signals is disturbing you in a long time sleep , sometimes you ate too much before sleep may disturb you and you will not wake up fresh , sometimes there is any weakness or deficiency in your body by which you will not wake up fresh , there are many persons which have a habit of proper time sleep if they disturb there time table of sleep or changes hour of sleep is also a cause for them for not waking up fresh , so according to my thinking if you over sleep or under sleep or you changes your hour of sleep or food timing may also lead to you a improper sleep and waking up dull instead of waking up refresh after taking a long time sleep.