Sufficient sleep improves performance
Even if you're a diehard morning exerciser, the world will not come crashing down on you if you decide to skip a workout for a little extra sleep. Frankly, if you have to choose between getting a workout in or getting enough sleep, almost always you should err on the side of sleep. (Although if that means you never exercise, we should have a little chat about your other life priorities.)
The reality is that constant sleep deprivation is pretty much the best way to ensure poor performance at the gym. When you're sleep-deprived, your coordination, endurance, and perceived exertion all suffer, making exercise feel more challenging, while opening yourself up to the potential for injuries. Being sleep-deprived also whacks out your hormones, making you more likely to overeat during the day while reducing energy expenditure (burning fewer calories). Which, together, is pretty much the blueprint for weight gain.
Sleep is your body's opportunity to rest, repair, and recover from everything you throw at it. Consistently failing to get enough sleep pretty much guarantees your entire life will suffer, even if you're loathe to admit it. Darin Hulslander, a certified strength and conditioning specialist and nutritionist, says, "Sleep provides mental, hormonal, psychological, and physical recovery. It isn't just a physical need; it's physical, hormonal, and neurological."
In other words, you could actually be a better athlete and a better human being if you just let yourself have another hour or two of rest in the morning.
It's not an either/or proposition, really
When it comes to sleeping in, I'm 100% for it. I hate mornings. You will not see me at the gym before 10am. Ever.
But I'm also 100% for fitting in exercise during your day, no matter what. It's not about sleep OR exercise, it's about sleep AND exercise. The two have a symbiotic relationship where one improves the other, and vice versa.
If morning workouts just aren't for you -- great! Don't do them. Use your mornings to guarantee you get as much quality sleep as possible, then go ahead and schedule in a workout during your lunch break or after work. You, and everyone else around you, will be better for the effort.