A. There are two reasons.
First you may not be getting as good a quality of sleep as you think. The dream cycle in sleep is important for regenerating your mind and ironing out the difficulties of the preceding day but many people have patchy sleep patterns and may wake up during the night, or don’t sleep well enough optimum dream activity.
This can leave them exhausted in the morning. The amino acid GABA, and the herbs Valerian, hops and passion flower, can all help. GABA is made in the body from glutamine, another amino acid. Combinations of these relaxing herbs (Valerian and GABA) and amino acids can help you sleep and wake up refreshed .
The second reason is that you may be waking up with a low blood sugar level. If so, you need to review your diet and intake of sugar and stimulants, especially caffeinated drinks such as tea and coffee deteriorate sleep . Cut these right out, along with chocolate, and go for a diet high in vegetables and fruits, high-quality protein such as lean free-range chicken and tofu, and wholegrains such as rye and quinoa. Be sure you also drink 1.5 liters of water a day.
References:  Komori T, Matsumoto T, Motomura E, Shiroyama T. The sleep-enhancing effect of valerian inhalation and sleep-shortening effect of lemon inhalation. PMID: 16857858.  Vlasta Březinová. Effect of caffeine on sleep. PMCID: PMC1402564.