At 6.16 Kṛṣṇa tells Arjuna who yoga is not for: the one who eats too much, the one who fasts too much, the one who sleeps too much and the one who stays awake too long.
At 6.17 he says who it is for: one who is temperate in food and rest, in work, in sleep and in waking. For that person, yoga destroys sorrow.
Eat properly. Sleep on time. It turns out the advice is older than Mummy.
The verses
BHAGAVAD GĪTĀ · 6.16नात्यश्नतस्तु योगोऽस्ति न चैकान्तमनश्नतः।
न चातिस्वप्नशीलस्य जाग्रतो नैव चार्जुन।।
nātyaśnatastu yogo ’sti na caikāntam anaśnataḥ
na cāti-svapna-śīlasya jāgrato naiva cārjuna
Yoga is not for him who overeats, nor for him who fasts excessively; not for him, O Arjuna, who sleeps too much, nor for him who stays awake too long.
BHAGAVAD GĪTĀ · 6.17युक्ताहारविहारस्य युक्तचेष्टस्य कर्मसु।
युक्तस्वप्नावबोधस्य योगो भवति दुःखहा।।
yuktāhāra-vihārasya yukta-ceṣṭasya karmasu
yukta-svapnāvabodhasya yogo bhavati duḥkha-hā
Yoga becomes the destroyer of sorrows for those who are temperate in food and recreation, temperate in their actions, and temperate in their sleep and wakefulness.
Going deeper
Chapter 6 is the Gītā's chapter on meditation, and before it asks for stillness it asks for ordinary balance. 6.15 says that a yogi who practises with a controlled mind attains peace. 6.16 then says who yoga is not for: the person who eats too much, the one who fasts too hard, the one who sleeps too much, and the one who stays awake too long. 6.17 gives the other side: for someone moderate in food and recreation, in effort, and in sleep and waking, yoga becomes the destroyer of sorrow. The next verse, 6.18, describes the mind that has settled on the self and wants nothing from objects. Seen together, the verses place everyday habits at the base of the practice instead of treating them as a distraction from it. Read the full chapter at chapter 6, and what the Gītā says about discipline follows the same thread.